1981 Eldorado Coolant Sensor / cruise control switch issues

[gtfcadillac]

After a couple of minutes when engine running coolant temperature light comes on dash and green cruise control light lites up on switch. When brake is depressed
both go off. When accelerator pedal is depressed both come on again. I changed the coolant sensor thinking maybe it was bad but the same thing still happens.
The car is not overheating, radiator temp is around 150 degrees. Can it be a faulty brake switch?

[TJ Hopland]

My 81 manuals are still packed from my last move so just some general thoughts only based on memory.

Normally when you get seemingly unrelated electrical interactions the first place to look is grounds.   That isn't normal behavior for the cruise light but at least there is a known electrical connection between the brake and cruise.  I would not think there is a connection to a coolant or engine temp but who knows every once in a while they did do odd things.  Maybe they kill the cruise as an additional notification of a temp issue.   

I don't think its a sensor issue but this would have at least 2 coolant sensors, one for the light, and one for the computer.   Its been too long since I worked on one of those so I don't remember where they were.   The light one would likely be a single wire and not an especially fancy weather proof connector.   The computer one would be at least 2 pins, maybe even a coax if I am remembering the correct car.    This would also have the engine temp light which like earlier models is a single wire sensor in the left rear corner of the head.  This one is more likely to interact with things because it often runs the buzzer / chime which is also tied to the key and seat belts.

Was it just at 150 when the light came on or is that as hot as it gets? 

How does the wiring look on this car?   Any signs of critters?   Aftermarket radio?   

[gtfcadillac]

Thx for commenting. the car has 45k original miles on it and is in very good shape except for some maintenance and electrical issues. The radio harness was cut, must have had after-market radio at one time however the original 8-track cb radio was in car when I bought it a couple of months ago. I had radio gone over and I bought original connectors and soldered them in place with shrink wrap over them. Everything works fine.

This issue occurs when car is running for a couple of minutes, not upon start-up. when brake is depressed both coolant temperature and green lite on cruise switch turn off. They come back on when brake pedal returns to normal position. That is why I was thin king maybe it is brake pedal switch?

[gtfcadillac]

BTW that's as hot as car got. Was a cold day outside here around 42 degrees.