Hi, I have found a odd issue with the temp gauge. As soon as the lights are turned on the temp goes up, turn them off and it goes back down. What electrical issue would cause this?

The temp gauge is now reading heat from the wiring loom...
Seriously now, it could be the resistor block thing in the back of the dash cluster. If possible, can you swap to another dash to diagnose it . It might be the temp wire is arcing across to the lights wires, strange one that ! I'm no sparky but process of elimination

no sparky here either, but as MM said, process of elimination:
Power-disconnect existing power  and going straight back to the battery +, then try with lights on/off
Earth- If its not power fault, do same with earthing straight back to battery -
Finally, if its neither of the above, it must be sender wiring: reconnect power and earth, and use your long length of wire straight to the sender to test.
 
 

May have found it, earth on radiator support panel.