On my son's truck you can shift from park without pressing the brake pedal. I changed out the solenoid in the dash thinking it would fix the issue. However you can still shift from park without pressing the brake. Does anyone have the wiring diagram or advice where to start looking?

 

The wiring diagram should be available here:

https://charm.li/Chevrolet/1999/

Just pick the right submodal and engine. You will have to dig around a little, but it's there. The issue is that 1999 was a transition year from the C/K series (GMT400) to the Silverado series (GMT800). You may need to know which engine/vin code to get the right manual.