Lost Manufacturer Filter?

I posted this under Product Types, but got no response at all. My apologies for re-posting, but am hoping someone might notice it here and have some clues...

Having finally gotten a custom product type field to show up in the index listings of Products by Category, I've somehow messed up the filter that should be applied when someone clicks a manufacturer name in the manufacturers dropdown: All products are listed instead of just those of the manuf. selected.

I've searched through the tools->Developers Toolbox for where that might be happening, and am at a loss. Have looked at all the pages in the index_filters folder, and that's probably the root of it, but I can't tell.

Anybody wishing to take a look may go see:
http://morsesalesandsalvage.com/catalog/
Drill down by category and all is well. Click a manufacturer and you'll get all 60-ish products instead of just that one manufacturer.

FYI the custom field is 'model_year' in the 'products_auto_extra' table.

Where does the manufacturer filter get created/populated/changed (whatever)? If I could just figure that out for sure, I'd know where to concentrate my efforts.

Hope some kind person will point me in the right direction. If there is some code I should post, I'd be glad to.

--eon--