I turn on my headlights and the oil and voltage gauges in my 88 GT quit working. WTH? Anyone got an idea?

Are those the 2 aux. gauges? There may be a common ground with the headlight motors or some other grounding problem.

You have a ground issue in your dash. When the lights are off the dash gauges are grounding through the light circuit. When the lights are also on there is no where for the dash to ground. Take the dash covers off. Turn the key on and the lights on. Wiggle the dash wires and the problem should show it's face.

Thanks, guys. I changed out the radio recently for one with a CD player and I bet I've introduced the problem with my ham fisted install. I'll pull it out and start checking.