To me, this sounds like bad contacts in the ignition switch. Like you said there was a temperature change that coincided with this problem. Perhaps the contacts are still making contact when he pulls out the key, having the light on. I've also had similar problems with me heater and key. it was cold one morning, and I started the car with a stiff key it started and the starter dis engaged well, ut the key's electrical position was not yet making contact with the 'run' connection, and I had no fan or radio till I turned the key a bit more CCW.

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450