EZ Wiring Mini 20 Circuit Installation

On my car, there was a separate plug at the firewall near the bulkhead connector that runs down to the transmission. Two prong...one straight and one at like a 20 degree angle to that. Those connect to the reverse light switch.

In your case, I'm guessing you have an automatic, so there is a combination reverse light/neutral safety switch on the transmission case. Two wires are for the reverse lights (feed power in one side and the feed out the other, doesn't matter which way), and one is a ground for the neutral safety switch. Use your multimeter and whichever one is ground with the trans in park is the neutral safety switch wire.