Ok, I know this is my first post here, and really hope to contribute, as much as get in return.
I'm having the same issue with my 2005 Ram. Cruise turns on, but it would not set for longer than 10-15 minutes, and got worse over time. Today, I noticed the two brake lights were stuck on, and I had to charge the two year old battery to get it started. Also, the third light mounted in the middle of the cab, responses properly with applying the brake pedal. Only the two rear are stuck.
The 3rd rear window light, in the middle of the cab is wired to the foot switch in the brake pedal, and the rear bed lights go through some pressure transducer in the break system, and feed back to the main computer. The main CPU see's the brake light on signal, and shuts off the cruise control.
If my reasoning is correct, where is the pressure transducer located in the breaking system of a 2005 Dodge Ram Hemi, and will I need to bleed the brakes after reinstalling a fresh on? How many hours at the dealer, to fix?