I have a 1980 280 SL that I'm restoring. Though I'm new to the MB cars there are many standards in all makes. Manufacturers for years won't let you turn the ventilating fan off so you don't die in their product. The 280 SL solves this in a very unique way. The fan switch, when in the off positon, closes the vacum source for the the fresh air\recirculating servo. The result does not allow you to have both the fan off and the fresh air off at the same time where even minor amounts of CO loaded exhaust would be bad. The less complex and cheaper approach is just to leave the fan running low all the time (the time honored GM approach). You might want to rethink being able to turn the fan all the way off unless you are positive there is some sort of fresh air compensation.