On my 93 300E, this sometimes happens. I never knew why but if you disconnect the battery terminal and reattach it, it goes away. Since it only happened about once a year, that seemed better than trying to find out exactly "why". You will probably have to enter the radio code again after you do this but it is a rather simple fix.

Don't know about your seat heaters since my car does not have them but maybe it will cure that as well.