I just got my '98 E430 back from the dealer today for the same problem. My sunshade was in the up position and frozen there.
When I took it in yesterday, the service advisor told me he had seen several W210's with sunshade problems. He said it was either the switch or the motor/actuator. It was not the switch. According to the shop order they replaced part# 210-810-02-20 ROLLER BLIND. Labor was 1 hr for electrical system test and 2.1 hours to replace part. All covered under warranty.
I think you need to head to the nearest dealer.