The digital display on my 2000 GP GTP stays at 50 degrees constantly. Unless it gets wet, then it can drop to -30. I've replaced the ambient air sensor behind the front bumper. Didn't work. Any ideas? It's causing the ac to act up (not blowing as cold as it should and going from cold to warm)

It probably has a short in the wiring somewhere, or a bad control head. Here's how to check for error codes: http://www.grandprix.net/forum/ubbthreads.php?ubb=showflat&Number=107192#Post107192

I checked it. It shows 04 for Ambient Outside Temperature Sensor Open. What does that mean and where do I go from here? Thanks

It means your air temp circuit is open. Either you replaced it with a bad one or you didn't plug it in all the way or there is a break in a wire etc.

Is there somewhere online that I can find a wiring diagram to follow so I can check the wires? Or if someone has a pdf I can give you my email.

Check your PMs.