ipreferpencils on "[Theme: Toolbox] Modern Fix for Styaing in Same Category with Post Nav With Multiple Categories?"

I've been searching the Forums and Codex for a few days trying about a half dozen fixes and I can't seem to find a solution that works. All of the solutions I'm coming across seem to be 2-8 years old as well.

GOAL:
I have posts with multiple categories attached to them.
I wish to be able to select a category, select a post within that category, then have the prev-next buttons on that post work within the selected category only.

PROBLEM: No matter what I do the prev/next links go to the next chronological post regardless what category I'm in.

SOLUTIONS TRIED: Adding "TRUE" to the prev_post_link and nav_post_link parameaters. Using different plugins. Trying to write my own code excluding the categories (sample below.) Toolbox puts prev/nav links are in functions.php file although I have tried all my solutions within the single.php file too.Tried switching back and forth permalinks to /%postname%/ and /%category%/%postname%/

CURRENT SETUP: WP 3.6, theme based on Automattic Toolbox 1.4, home page static, blog page "drawings" with 14 categories. Permalinks set to /%category%/%postname%/

WEBSITE: http://www.newenglanddrawings.com/drawings

CURRENT CODE I'm Trying...

I'm NOT a programmer by any stretch of the imagination, however after reading the codex on the previous_post_link and next_post_link I thought this solution would work. I have 14 categories, so I apologize if this is long. This is what I have in the functions.php page of my theme....

/**
 * Display navigation to next/previous pages when applicable
 *
 * @since Toolbox 1.2
 */
function toolbox_content_nav( $nav_id ) {
	global $wp_query;

	?>
	<nav id="<?php echo $nav_id; ?>">
		<h1 class="assistive-text section-heading"><?php _e( 'Post navigation', 'toolbox' ); ?></h1>

	<?php if ( is_single() ) : // navigation links for single posts ?>

	<?php if ( is_page('drawings') ) {
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE );
	}else if ( in_category('3') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '4,5,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('4') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,5,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('5') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('6') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('7') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,8,9,10,13,14,17,18,19' );
	}else if ( in_category('8') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,9,10,13,14,17,18,19' );
		}else if ( in_category('9') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,10,13,14,17,18,19' );
		}else if ( in_category('10') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,13,14,17,18,19' );
			}else if ( in_category('13') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,10,14,17,18,19' );
	}else if ( in_category('14') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,10,13,17,18,19' );
	}else if ( in_category('17') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,10,13,14,18,19' );
	}else if ( in_category('18') ){
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,10,13,14,17,19' );
	}else if ( in_category('19') ) {
	previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'toolbox' ) . '</span> Previous Drawing', TRUE, '3,4,5,6,7,8,9,10,13,14,17,18' ); }
	?>

		<?php if ( is_page('drawings') ) {
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>' );
	}else if ( in_category('3') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '4,5,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('4') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,5,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('5') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,6,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('6') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,7,8,9,10,13,14,17,18,19' );
	}else if ( in_category('7') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,8,9,10,13,14,17,18,19' );
	}else if ( in_category('8') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,9,10,13,14,17,18,19' );
		}else if ( in_category('9') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,10,13,14,17,18,19' );
		}else if ( in_category('10') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,13,14,17,18,19' );
			}else if ( in_category('13') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,10,14,17,18,19' );
	}else if ( in_category('14') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,10,13,17,18,19' );
	}else if ( in_category('15') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,10,13,14,18,19' );
	}else if ( in_category('18') ){
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,10,13,14,17,19' );
	}else if ( in_category('19') ) {
	next_post_link( '<div class="nav-next">%link</div>', 'Next Drawing <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'toolbox' ) . '</span>', TRUE, '3,4,5,6,7,8,9,10,13,14,17,18' ); }
	?>

	<?php elseif ( $wp_query->max_num_pages > 1 && ( is_home() || is_archive() || is_search() ) ) : // navigation links for home, archive, and search pages ?>

		<?php if ( get_next_posts_link() ) : ?>
		<div class="nav-previous"><?php next_posts_link( __( '<span class="meta-nav">&larr;</span> Older Drawings', 'toolbox' ) ); ?></div>
		<?php endif; ?>

		<?php if ( get_previous_posts_link() ) : ?>
		<div class="nav-next"><?php previous_posts_link( __( 'Newer Drawings <span class="meta-nav">&rarr;</span>', 'toolbox' ) ); ?></div>
		<?php endif; ?>

	<?php endif; ?>

	</nav><!-- #<?php echo $nav_id; ?> -->

I would appreciate any help.
Thank you.