I worked at a stereo shop for 10 years and put headunits in some strange places. The only real concerns are heat from the amp in the radio and the CD player skipping. If you run after market amps or get enough air to the back of the radio, you will be fine. The angle you would have to mount it at in the secret compartment would not allow the vibration dampeners for the CD work properly. If you use your Ipod or the radio for most of your music, obviously, this won't be a problem. You could mount the radio in the trunk to the left of the spare. This way you could retain your secret compartment.

 

Not a bad idea, I might see about doing that and just running a wire to the console... I'm not sure, but i have time.