I'm not in the US, but my car didn't have an ISO plug.
I had to get a Ford to ISO convertor (About 15 UKP, including the speaker connectors)

However, as I have 2 head units, I've spliced another ISO (with just the power wires) into the convertor (Actually, it was spliced for the Mk1, I just found another ISO plug and stuck it on the wires for the Mk2... When I say spliced, I mean all nicely soldered and heatshrunk... =)

So see if you can get a convertor before chopping the wires off.

Jazz
(List 112, Mk2 12 gig #40. Mk1 for sale 4 gig #30, apply within)