The PCB in the GMLAN boxes is a "cheap Chinese" generic /simple/not copyrighted circuit. A basic AD/DA. Any sophomore electrical engineering student could design and build one. Might even be simpler than that. A few wires and a relay or transistor would deal with the power requirements, the SWC is just a 1-wire signal.

 

How do I test if the SWC is working?
The new unit I got to make SWC work will not link to the car.
It connects to pin 11 on one connector and pin 14 pin the other.
The white black wire should have 10 volts and the dark blue goes to the SWC. The tech Guy I was speaking to said if I put the multimeter on pin 11 and pin 14 and touch the steering wheel controls the voltage on the multimeter should fluctuate. I'm getting nothing. THe white black wire that is supposed to have 10 volts has nothing. When I took the stock radio out everything was working. Thanks Scott

 

If I remember right the 10 volt ref is provided by the HU

 

What does HU mean?

 

Head Unit…the radio in the dash waterfall.