1999 cummins 5.9 Jake brake

OP here and Wanted to give an update. After checking voltage at the switch and air supply and physically moving the jake brake I determined it was an electrical issue. No power at the solenoid. Found a wire coming from the chassis fuse box that had an inline fuse that was blown. Changed out the fuse and it powered the solenoid and brake seemed to be working. Took for a test drive and 10 minutes in no jake brake and fuse blown again. Climbed underneath and traced all 36+ feet of wire and no signs of damage. Decided to change fuse and give it another go. Went to open the fuse holder and the butt end connector fell apart in my hand. Crimped in new fuse holder and made our Thanksgiving trip and the brake worked perfectly. One $3 dollar fuse holder with a bad crimp but at least now I have a better idea of how the system works.

My 98 has the same setup, as above I would be checking to see if the brake is free and check the cables and air hose.

My exhaust brake is very accessible in the engine compartment of my 94 Bounder with the 5.9 cummins. Mine has a small air compressor with a solenoid and an air line going to the valve on the exhaust pipe right after the turbocharger. I have a switch in the dash to activate or turn off.
If mine stopped I would first look at the voltage supplying the solenoid, check the air line, physically check for free movement at the valve and ensure switch is good. It is also connected to the transmission and there is a wire that connects both. Hope that is a start. Doug