Sounds like something is earthing on the lights circuit, or the lights circuit is grounding on something it shouldn't. I know you've checked the earths, but I imagine there is a bad/unwanted contact somewhere.
With the temp gauge, you didn't mention whether it was working before you changed the blue sensor?
If it was working before, there's a black sensor next to the blue one which sends the temperature signal to the gauge on the clocks - make sure you didn't disconnect it when doing the blue sensor, or disturb a wire. With the wiring in these cars being 20 years old +/- it's possible that some wires may have been hanging by a thread, until they were nudged and broken.
If the temperature has always been cool, check the thermostat is coming on when it should (i.e. the bottom radiator pipe should be stone cold until the engine is warm). If the bottom pipe warms straight away, you need a new thermostat as it's stuck open.
If the thermostat is okay, the next two things to check would be for airlocks, and whether the gauge in the dash is working
