Chasing electrical issues

[theWoodsmen]

So to start off I recently bought a 14 gmc sierra denali from a sketchy car lot (first mistake) and there were a few red flags that I should have noticed but didn't, long story short the engine had been taken out and put back in. 

To begin when I got it the cigarette lights would spaz out when I rolled up the windows, I fixed that with a braded ground cable on the driver side going to the fire wall, but now since I installed that I'm having a temperature gauge issue where after it runs for a little while off and on the gauge starts to read a lil high then drops down to 0 and the electric fans kick on and I have to unhook the positive battery cable to get it to reset but I'm not sure where to go from there.

Thanks, William