If you have a JVC deck in it already, your factory radio plug may have been cut--which would put you out of the running for a FAST harness.
Pull the deck and check behind it.
Also, what year is the vehicle?
You could just tap into the speaker wires, give it some power and go, you know.
Derek Tircuit
Product Specialist
Bazooka Mobile Audio
1-800-THE-TUBE
derekt@bazooka.com