High Fan Speed Won't Work

[renok7]

I own a '93 300E 4 matic. With the ignition key on instruments the high fan setting works normally. However, when the car is running, the high fan setting won't work. My mechanic thinks that the blower motor regulator switch is defective but he isn't sure since the high fan setting works normally when the car in not running (i.e. instruments only). Before that he had replaced the ignition switch and tried a new push button control but to no avail. Any input would be greatly appreciated.

Thanks