The wiring harness should not cause overheating, but it may cause a false reading on the meter. If you suspect this, check temperature around the engine, and especially at the thermostat housing with a non-contact pyrometer.

Bad grounds between the engine and body can cause false meter readings. If this is the case, there may be a difference in readings when the electrical consumption changes.

However, if your radiator is original, it could very well be the problem.