Several questions in one...  We are a car audio business and would like to add an additional custom car specific search option to our site.  It would be nice if it appeared under, and in addition to, regular search box on left pane. 

I think I would need a custom table with the following fields: "year", "make", "model", "part#" since one part# fits multiple vehicles and each vehicle has multiple part#'s it can use.  The part# field would be my link to Item/Products. I'm not quite sure how to set up and create custom table for this.

For search, I would like dropdown for "year", then based on year selected all "makes" with a record for selected year would appear in the next dropdown.  Then based on "year" and associated "make", the next dropdown would show all "models" available.  Then I would like all "part#'s" that fit criteria to be displayed as from a regular search with the option to buy.  So I don't think you'd actually be searching item table/fields but a custom table and then linking to appopriate items to display.