Wiring Help: Aftermarket GPS navigation install

I have a wiring harness and mounting kit from Crutchfield to replace the shaker500 with the new double din deck I am installing. I have all the wiring prepared with the exception of three wires.

1. Speed Control
2. Reverse Sense
3. Parking Brake

The reverse sense and parking brake are easy but I don't want to have to run a wire all the way to the trunk and tap the reverse light. Some of these wires must be coming from under the dash already. It is a pain to route a wire all the way back there. The speed control is a complete mystery because I can't find a wiring diagram for a 2008 GT-500 I assume I could get by with a GT wiring diagram, neither are readily available online.

A little help please.....