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The RAP relay feeds the sunroof module, windshield wiper switch, turn signal/malfunction switch, door lock/window switches, rear washer pumps, rear wiper switch and auto trans floor shift control.
At the base of the steering column, just to the left is a light gray 22-pin connector.
Pin B7 is a yellow wire from the body harness, and a black wire going to the windshield washer/wiper/multifunction switch. This is the RAP circuit.
Laying upside down might make this hard to see, just make sure that the connector end coming from the body harness has to be yellow, while the one going to the switch is black.
There is another yellow wire coming from the body harness, but DO NOT TAP into this wire, this yellow wire will have a gray wire on the other side of the connector going to the switch.
It will be easy to test with a test light, or voltmeter.
That is the closest place, since the radio's RAP function is via a serial bus circuit.

There is another place if you have a sunroof. Run the wire over to the driver side A-pillar. Remove the A-pillar molding (remove the small cover, the screw underneath, and just pull it off). If you have a sunroof, tap into the yellow wire in the wiring harness running up the A-pillar.
If you go this route, make sure it is a solid yellow wire, the yellow/black striped wire is the Onstar LED signal for the green LED.