I'm planning to tear into my 72 350SL this summer to get those old style heater controls operating properly, too. Mine won't operate the solenoid (three-way) valves for some reason, no electric signal to the valve. As to those controls, I understand there is a cable adjustment for the vent flaps and a lever clamp which sometime slips. My driver-side lever won't go all the way up. I think it has slipped a bit.

My indy mechanic told me that a common "fix" for heater controls used to be to loosen the clamp so the flap would not bind or jam. Maybe you should check this, but I think you need to get into the console.

I believe the slides are cable controlled to the vent flaps, but there is a micro-switch attached to each heat lever that operates a pair of electric three-way valves that open and close the heat valve on the firewall.

230/8