Richard / E320

I've got a friend that is having some sort of electrical issue with his 2005 CLK500. Details are sketchy but he says that while he is driving with the heater in the 'off' position, it will turn on by itself and he cannot not turn it off. The fan runs at about mid-speed and at times will turn off, but frequently will not turn off until he turns off the ignition. Upon re-start, the heater will usually be off but then start the whole process again while driving.

Any suggestions on what to look for, what might be shorting out or what part might be giving up?

Thank you,