Hi,
Try to also check the condition of the Blower. Locate the carbon brushes and see if it is worn out. A common problem of this is that when it is quite worn out, it is noisy (squeaky sounding) and that it does not spin as fast as when it was new.
If you think it is worn out then contemplate changing the entire system to the updated one since the blower is not easily repairable. You may change the carbon brushes but it will not last long since aftermarket carbon brushes are not compatible to the one used by the Bosch blower. Also, I think the old blower is a hard to find item. Unless-again you may be lucky to find it in a salvage benz shop.
But if the blower is fine, just invest on that blower regulator. If it last 2-3 years then it is worth it.