Because of large number of databases that I am selling, and because each customer have different requirements in integrating my database in their website, there is NOT possible to make ONE plugin and give it to every customer purchasing a database.
Several free plugins for WordPress do exist (example WooCommerce YMM search) but it can be used only for online stores made with WooCommerce and the only car database from me that works with it is the American Year-Make-Model (or equivalent 3-columns databases for other markets), unless you edit plugin codes to make it suitable for 4-columns format used by European databases. Someone told me that is using Formidable Forms plugin.
Although I made this WordPress website myself without any help from third party people, I do not have sufficient experience in PHP coding to make custom plugins for other people.
Most of my customers are web designers or have a webdesign team so they made themselves the required scripts, they did not asked me to provide any plugin or scripts. If you do not have own web designer I can connect you with my friends in web design, they can help you making custom PHP scripts according your requirements (you need to pay them separately than payment for my database).
If you found other WordPress plugins that works with any of my databases, or made yourself a plugin, can you share with me for my other customers?