Temp gauge not accurate (high readings)

[Nioreh]

I've lately noticed my temp gauge is reading higher than it did before. It has always read a bit high, but now it's very high, ever since I got the radio and lambda ECU running. The gauge is affected a lot by things like turning the headlights on, or the dash lights, making it read even higher. This makes me think it's an electrical problem. Any extra electical load seems to make it read higher. I don't think it actually gets hotter than normal, as the fans kick on and cycle normally, and there ar no other signs of it overheating. But it still causes a bad feeling driving the car, so I'd like to fix it.

I am not super familiar with electronics, but as far as I can tell, this could potentially be caused by a weak ground connection on the gauge, which it uses to compare the signal from the sensor? It this a correct assumption? If so, where is the temp gauge grounded? Any usual suspects?

I am planning on trying to fix it this weekend. Any experience or tips on this issue is appreciated!

Oh, and I also have never had the battery light functioning. Car charges OK, so not sure how that all works. Could it affect the gauges?

[Bitsyncmaster]

Try moving the inertia switch ground wire to frame ground. That 6 amps of fuel pump power causes a voltage drop on the ground wire for the instruments.

[Nioreh]

Try moving the inertia switch ground wire to frame ground. That 6 amps of fuel pump power causes a voltage drop on the ground wire for the instruments.

Thanks! I will try that, doesn't look too difficult.