Works perfectly in every position until that engine is running.

The only other accessories is a electric brake assist for a trailer, a trickle charger that the previous owner had in it for what reason I don't know and some cheap fog lights (that I need to remove) that were on it when I bought it.

The problem is definitely something cyclical seeing that the signal bounces at a steady rate back and forth. It's not random, but like every second on the second. Weird.

I had a external sat radio in it and thought maybe the radio was bad so I went to Best Buy and bought one of the ones that plugs directly into the new head unit, but the result is the same. So the old external radio has become my test radio. I unplugged the alternator yesterday, plugged in the test radio, placed the antenna on the roof a good 6 feet from the stereo antenna and stood outside the truck holding the test radio and the signal still bounced back and forth. I have a hunch it's the alarm but I can't prove it.