1986 300sdl - question on center vent

[ruchase]

I've finally had it with the CA weather and decided to try and sort out the A/C issues with my 300sdl. I've had the car for about 5 years and almost all maintenance has been performed on schedule. I say almost, because the A/C is the only thing that's been neglected. When I bought the car, the A/C blew moderately cold. However, the center vents would work erratically and then the system would shift to defrost mode; at that point the mediocre A/C would no longer cool the cabin. This wasn't an issue because I've always had a second car with working A/C so on really hot days I just wouldn't drive it.

Well my second car's A/C is now out of commission, so while I get the funds together to have the entire system rebuilt, I'd like to at least see if there's an easy fix for the SDL's center vents so I can also have this A/C charged. It really annoys me that the MB engineers designed such a convoluted and overly complex ACC for these cars in U.S. trim.

Is there a simple way (without removing dash) for me to rig the system so that the center vents are always open? Down in SoCal we hardly ever need defrost. Thanks!