Went through this exact same thing with my '99 Silverado. After installing new front speakers (cones dry rotted) AND a new head unit, still had the same problem...everything worked but the sound! Got so mad I SLAMMED the passenger door and the radio started playing. Slammed it again and it quit.

Turned out to be the rear door speaker wire was pinched in the speaker frame and finally rubbed through. These units are "above ground" so all it take is either wire to touch the chassis ground and the radio completely shuts down the audio.

Radio?
 

I also have a 99 what type of radio will fit? What type of aftermarket radios get excellent radio channel reception? I installed one in my old 91 chevy and the station signals suck##.

My 99 I wish I could get better reception with it also for some channels i like.

Here ya go

https://www.truckforums.com/forum/ne...newthread&f=11

Update
 

Ok...stereo worked fine yesterday. Turned the fader and all speakers are putting out sound. Today...not so much. It died again. I am left to believe there is a relay I can't find.

ETA: I ended up ripping out the stereo to check all of the connections from the backside and it appeared to be working fine. HOWEVER, my roommate thought it was trash, so the stock unit is no more. Next project: pick up another head unit and start again.

I think you got a bad or a few bad wires or a bad plug.