My father in law has a 1991 300E that simply won't start in cold weather. The starter turns but the engine will not turn over. The problem has existed for some time and the car has been serviced numerous times. The only way it starts is if we hang a light bulb in the engine compartment overnight. This apparently keeps it warm enough to start normally the next morning.

Bad sensor?
Any help/info would be appreciated.

Thanks,
-Jeff