Temperature gauge moves when lights are on

[evil1kustoms]

Just replaced the radiator on my 89 bronco II that i just got, When you turn the lights or the ac on the temp gauge will jump up and down. Not sure how long its been doing this or if it just started. I'm guessing a bad ground somewhere but does anyone know which ground that would be or if its something else?

 

[Jspafford]

Have you simply checked the connection to the sending unit for the guage? It's just a press over terminal with a black boot.