But when I took out the factory radio, I found two harnesses plugging into the factory radio! But my aftermarket harness can only hook up to one of them! Is the other harness unused?
Did it have premium audio or something? Maybe the other is an amp?
On a neighbor's 95 Ford Explorer, she had me disconnect the radio on to troubleshoot a dead battery issue, has two connectors also. I was simply disconnecting it so I didn't do any harness investigation.
The second, smaller plug is for the factory amplifier on Moon & Tunes / Sun & Sound cars. Don't worry about connecting it, it's only needed with the factory radio.
Here is the pinout of the larger harness for cars with or without external amplifier, if you need it:
03 Vibe base. Born 10/14/2002 06:07 AM
Auto, Moon & Tunes, power package. 143k
Neptune/dying clearcoat/primer grey.
That second black connector is for the remote 6 disc CD changer that was installed with the navigation systems on the Vibe and optional on others. I have it on my 03 GT and when I installed my son's base radio in it, since the stock nav screen was dead, I plugged in the second connector into his radio and the cd changer works just fine. But all you need is the adapter for the first connector to hook up an aftermarket radio.