Adding fields and tabs

Parts of this have been asked before, but I can’t get the suggested solutions to work.
I want to add custom fields for the details and specifications categories of my listings.
I managed to add a custom detail field by adding the following two filters to my functions.php:

add_filter( 'auto_listings_metabox_details', function ( $fields ) {
    $prefix = '_al_listing_';
    $fields[3] = array(
        'name' => __( 'Test-Detail', 'auto-listings' ),
        'desc' => __( 'A custom description', 'auto-listings' ),
        'id' => $prefix . 'test-detail',
        'type' => 'text',
    );
    return $fields;
} );

add_filter( 'auto_listings_details_array', function( $fields ) {
	$fields[ 'Test-Detail' ] = auto_listings_meta( 'test-detail' );
	return $fields;
} );

But when I try to add custom specifications, it only shows up in the backend. The frontend doesn’t display the value I entered.
Here’s the code I’m using to generate the custom spec:

add_filter( 'auto_listings_metabox_specs', function ( $fields ) {
    $prefix = '_al_listing_';
    $fields[3] = array(
        'name' => __( 'Test-Spezifikation', 'auto-listings' ),
        'desc' => __( 'A custom description', 'auto-listings' ),
        'id' => $prefix . 'test-spezifikation',
        'type' => 'text',
    );
    return $fields;
} );

add_filter( 'auto_listings_spec_fields', function( $fields ) {
	$fields[ 'Test-Spezifikation' ] = auto_listings_meta( 'test-spezifikation' );
	return $fields;
} );

There’s probably an error in the first filter as that’s the one I tried to reverse-engineer from the previous “details” filter.

And how would I go about adding my own tabs? I need to sort the available specs into smaller sub-categories instead of listing everything under “details” and “specifications”.

The page I need help with: [log in to see the link]