It sounds mad but when I turn on the headlights, the coolant temp gauge reads slightly warmer than otherwise and the (stock) radio goes from completely off to off but showing the word: CODE. A periodic popping sound through the speakers accompanies these symptoms.

Any idea where the headlights, radio and coolant gauge might cross wires?

I am no electrician, but this sounds like classic 'earth feedback' or poor earthing symptoms. The problem is in the millons of wires under the dash and console, for sure. There are lots of systems earthed there, including two very important multiple earths on the vertical body bracers each side of the aircon unit. These are VERY hard to see but just possible with the lower dash removed and other bits pushed out of the way.

Also there are earths on the tunnel, also findable once the console is out.

Greg