Ok, I'll let's try this again.....
How good of a php coder are you? What are you comfortable doing to your site? What modifications/installations have you done successfully?
In my experience, you'll need to know how to add columns to some tables in your database, add a new page to your site for the search itself (or a new infobox like I just did), and add a search results page to your site. Not sure if you've gotten that far and gotten stuck, or if you'd just posted a "Gee it'd be great if this was an easy contribution" post here.
Where's the contribution you mentioned? I thought I'd seen something somewhere, but it used the established categories from the products table and appear to be as usable as the application guide you pointed out from NAPA.
I built a site that did a search for wheels by vehicle at my previous job (
www.wheelstudio.com) which used a vehicles database and searched for products based on custom fields in the product table. I *just* got done making something similar to the link you sent on NAPA using AJAX to search for parts by vehicle year, make and model. Unfortunately this is something I can't share since I am working for a company and not myself (I hope you understand why I can't share it? I only mention it because it took me less than a week to make it from scratch - a good chunk of data included in that timeframe).
FWIW, I did a search on "AJAX and PHP" on Google and went step by step in figuring out how to build the search app for my site.
Good luck, and remember an ounce of sugar is worth more than a pound of salt...