avh-p4900dvd will not power up

There should be a yellow wire you need to hook up for the switched power.

 

"getting anything out of it at all"

You have no power from the yellow circuit? Trace where the yellow is tapped into and see if the fuse is blown. You could always run a new wire directly to the battery as long as you use an inline fuse.

 

Try grounding your HU somewhere else besides using the black harness wire. You can also just connect the red & yellow wires to see if your HU will turn on that way. If it does, figure out which 12V wire is problematic.

 

A 2004 Silverado has a Class 2 Databus system. If you disconnect to stock radio, you can't get 12-volts on the accessory wire. The vehicle computer communicates with the stock head unit and only turns on the ACC if it sees it.

You have to get the proper Class 2 Databus adapter to have the accessory (and other vehicle functions) operate properly. Without it, you need to run a new accessory, but you will loose the RAP function and possibly the air bag.

The proper adapter is about $99.