Modifications for the Product Category Dropdowns WordPress plugin:

Horizontal view

This modification will display all drop-down selects in one line and will display one select per line on mobile (responsive) view.
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download modified files and instructions : horizontal_view.zip
download the Product Category Dropdowns 1.0.0 plugin with all files modified: pcd_1.0.0_with_horizontal_view.zip

Start from selected category

This modification will display child categories of some selected category instead of the root categories in the first drop-down select.
You will have to specify the category ID in the .php file like this:
protected $_startFromCategoryId = 16;
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download modified files and instructions : start_from_category_id.zip

Only root categories

It will display only the first drop-down with the root categories.
After selecting the first drop-down it will redirect to the category page.
download modified files and instructions: only_root_categories.zip

Exclude some root categories

It will not display some categories in the first dropdown select.
You will have to specify category ids in the .php file like this:
$excludeCategoryIds = array(15, 56, 12);
download modified files and instructions: exclude_some_root_categories.zip

Limit number of levels

This modification will submit after selecting the second dropdown select.
You can specify the number of levels in the .js file like this:
levelLimit : 2,
download modified files and instructions: limit_number_of_levels.zip

Only categories with in stock products

It will display only the categories that have products available for purchase (In Stock).
download modified files and instructions: only_categories_with_in_stock_products.zip

Display empty categories

It will display categories with no products in the dropdown selects.
Instructions: display_empty_categories.txt

Category sort order

It will display categories in Category order instead of ordering alphabetically by name.
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download modified files and instructions: sort_order_category_order.zip

Custom dropdown titles

With this modification you can specify custom titles for the first, second and the third drop-down selects.
Also it is possible to display different titles depending on the first selected category.
You will have to specify the category titles in the .js file like this:
var selectTitles = ["-- select category --", "-- select sub category --", "-- select sub sub category --"];
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download modified files and instructions : custom_dropdown_titles_for_category_id.zip

Disable auto search

To make it not search automatically when the last category is selected.
User will have to click the Search button to see the results.
Replace the line:
this.submitCategory(cId);

with:
// this.submitCategory(cId);

in the file:
wp-content/plugins/product-category-dropdowns/view/frontend/web/main.js

Display default selects on page load

It will make three drop-down selects always visible.
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download modified files and instructions : display_default_dropdowns_on_page_load.zip
NOTE: If you need it for make and model selection check this plugin:
https://wordpress.org/plugins/ymm-search/

Display image for selected category

It will display an image below the drop-down selects for the selected category.
You will have to specify image file names for category ids in the .js file like this:
images[91] = '3157.jpeg';
images[89] = 'Car.gif';
and then to upload the images in some folder on your website.
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download modified files and instructions : display_image_for_selected_category.zip

Display product tags in the last dropdown

It will display product tags in the last drop-down select after selecting all categories.
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download modified files and instructions: display_product_tags_in_last_dropdown.zip
download the Product Category Dropdowns 1.0.0 plugin with all files modified: pcd_1.0.0_with_product_tags_in_last_dropdown.zip

Display products in the last dropdown

It will display an extra drop-down select with products for the last selected category.
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You can buy this modification on the page: /pcd-products-in-last-dropdown.html

Products of the same category

It will display a drop-down select on the product page with other products of the same category.
So that the user can switch the product.
This drop-down select can be added to the product page with a shortcode.
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download modified files and instructions: products_of_the_same_category.zip