I have the three pod gauges. oil psi, water temp, and trans temp. I have the power wire to turn on gauges in tap with the radio wire... The trans temp gauge has backround lite up 24/7, seeing it as how its a blackout gauge. When I turn on the headlights, the other two gauges light up but the needle on the trans temp lowers about 20 degrees.... wtf.. turn headlights off, needle jumps back up

 

The voltage to your instrument panel, and radio lights changes with the variable resister on your headlight switch. Maybe that has something to do with it.

 

it was a ground back feeding through my system. I grounded someplace else, although, the needle will move a TINY bit still when headlights are turned on.. but its fine I guess