I have a 2003 34FDDS, and My solinoid went out too. The unit is not rated for continuous duty and if you are parked and jacks set, the air tank is empty. If you set something on the stearting wheel like a bed comforter, it will depress the air horn bar on the stearing wheel and activate the solinoid even though the ignition switch is off. You won't hear the horn as the air tank is empty, athus the solinoid will burn out after a very short period of time.
The solinoid is located at a level about the top of the CRT backup camera and is on the fire wall about 6" to the right of the CRT face. WRV tech replaced the unit annd I watched. It a one handed job and long arms are a must. It is very difficult to reach from either side due to all the wiring and vent tubing etc that tends to block access. I took the tech, who had obviously done this job befor, about 1.5 hours whith a lot of 4 letter words to help himself along. I'm a handyman and I wouldn't do it. Good luck!
Alpine 34 FDDS, '06 Jeep Rubicon