Modifying Search with Dropdowns

[enlighten_me]

I haven't worked in PHP in a while, so I'd be extremely grateful is someone pointed me to the right file to work on.

I'm developing a car products cart, and would like to create dropdown boxes for make and model. Once the model is selected, the make/model is forwarded to the search function. I'm having some difficulty locating the search function(s).

Also, if possible, is there an option to just search through the titles or tags?

Thanks!

[enlighten_me]

I've grepped a number of files:

search_header.php
tpl_search_header.php

I also see a number of search options to in header_php.php. Search by keyword, search_in_description, categories_id, etc.

What's dfrom, dto, pfrom, and pto?