that's weird. one thing shouldn't interfere with another (AUX mode should stay aux mode) until manually switched.

which sirius unit? where is your antenna?

Sirius will drop the signal when going under bridges, etc. especially older generation receivers. However, it should restart when the antenna has a clear view of the ski.

Regarding the mode switching to FM, that should have nothing to do with the sirius unit dropping the signal. for instance, when i switch to AUX, I can turn on and off my sirius unit (getting a thump) but even if there is dead space in the circuit, it won't switch out until I hit the mode button. I can turn the sirius back on, and it works fine. I don't know if this is a function of the business radio as I have the Nav setup (DSP)...

Who installed the aux and sirius radio? where is the sirius radio drawing its power from? have you tried powering down and repowering the sirius radio when you lose the signal before it switches to FM to see what happens?