temp. gauge in the redline, on board comp. says rear lights not working.

[asd434]

96 cherokee with the 6 cyl. the other day, the temp gauge shot up into the red out of the blue. It would go back and forth until now, it finally is stuck on the max. I brought it to the dealer, I believe he said the coolant sensor was working fine and it might be the gauge itself. he did not fix it. I had multiple problems to also fix.
when I got the truck back, I noticed that the steering wheel radio controls did not work and all the dash lights were dim. the dash lights now flicker really bright at times. I'm not sure if I should fix this truck or trade it in.
the mechanic also fixed the license plate lights which had a rotted wire. the code is still in the computer screen to fix the lights, flashing red. how do I get rid of this? thanks for any help.....