Brake dash light and 5th wheel release

I have a 2007 387, when you set the brakes, a red indicator light should come on. It's only been working intermittently lately.

I have an in cab 5th wheel release and when the light fails to come on, the fifth wheel does not want to release, even manually.

Is there a sensor that links the brakes to the 5th wheel release????

The light seems to work fine when the engine is cold, soon as it warms up, if I have to set the brakes it won't come on....Also, when the light fails to come on, I can't idle up the truck.....

I should add that the brakes work normally...