I've owned a '80 300SD since new and sympathize with you; this climate
control is the most difficult "system" in the world to fix.
I've looked at the wiring schematic and have a suggestion or two.
#3 and #9 fuses are connected via the double pole relay, so there's
reason for both to blow.
I first suggest you temporarily disconnect the electric auxiliary water
pump as a test; it may be shorted and blowing #9 fuse. If that's
excluded and the fuses still blow go to step two.
Then I suggest you temporarily disconnect the electric auxiliary fan as
a test; it may be shorted and blowing #9 fuse. If that's excluded and
the fuses still blow go to step three.
DEF causes the blower to run at maximum speed and heat, if the fuses
also blow when AUTO HIGH is pressed and the TEMP wheel is set to say, 60
degrees on a hot day then I'd check the blower motor. I suggest this
because the blower motor runs at maximum only at these times. The motor
is located behind the glove box, which can be removed by removing its
plastic rivets. If the blower motor PROVES to be OK, despite all its
use, then I'm afraid the servo is the most likely suspect, especially if
it's more than 7 to 10 years old.
Unfortunately one just doesn't know; it's too complicated to comprehend
and the repair becomes a guessing game. That's the problem that
everyone, not just your shop, has with this old system. Unfortunately
the fellows who understood it have mostly retired from wrenching.
I personally don't touch it, I just leave it on Auto and hope it runs