Two current questions, among many, and would like your ideas.

It's an '85 190D I've owned since new, driven and cared for by a possessive wife that never let me drive it. She serviced it at dealers throughout her life, and I've learned my lesson. I've spent the last 6 weeks correcting their screw ups and games, most of which I've fixed. I'm in process of 'restoring' and upgrading and among the MANY problems, I have two current electrical problems, a temperature gauge that won't work right and an ignition switch that's screwing up the warning lights.
On the switch, out of the clear through the many removals of the instrument cluster (15 through today,) the seat belt warning light decided to function opposite its normal operation, when the belt is off the light is off, buckle up and the light goes on. The other warning lights are quite delayed in going off, and don't until the engine is revved up. Not sure if it's in the switch or the alternator isn't putting out enough to shut them off. I'm uncertain how it functions. How to repair switch; replace or can it be disassembled, cleaned and repaired?

The big problem is the temp gauge. While I was returning the car home a few weeks back, the temp gauge started acting erratically and finally it quit working altogether after making a buzzing sound a couple of times after start-up. I've replaced the three coolant sensors and no change. I replaced the gauge and the new gauge reacts slightly but barely comes off the turned off position so it's getting some current. There is continuity in the line from the main sensor of the three and the resistance is minimal, about what you'd expect for the length of wire. When the connector on the main sensor is shorted the gauge goes to the top.
There is no corrosion on the various connectors and I've removed and cleaned the ground connections on the grounding block behind the cluster. The fuses have all been replaced and there is no corrosion on the connectors under the electrical center. Everything looked OK.

The big question I have, besides why it won't work, is how those sensors work. I replaced the ones that were in the block (one between #1 and #2, and the other by #3 cylinders,) with the only one listed as the coolant sensor. The one that runs the electric fan I understand. That fan was turning on when the ignition turned on and replacing it solved that. No effect on the gauge naturally. I don't know if the other sensors were previously replaced but they are different and carry different numbers, one a Bura, the other a Bosch that says 120°. I could locate nothing on EPC. The lines to both sensors unite and run to a relay on the left fender and the line then proceeds to the electrical center. No corrosion on that relay either. Are the sensors working in tandem or are the some how fighting each other, and does it require different sensors? I wouldn't think so, but why two? Nothing in the wiring diagrams I've seen.

I'm at a loss and fixin' to replace cluster so if there are ideas, I'd love to have them. I do have pictures.