There must be a virus going around. I have the same symptoms on my '80 300D. After I replaced the servo because it was killing my battery, things got worse. Now my LO/HI/BI buttons do not work. Only the DEF and though the compressor engages, it blows hot air. The fans go off and the compressor disengages if anything subsequent to DEF is selected. On top of this, my engine does not shut off immediately which leads me to believe I have a vacuum leak.
And all this because I replaced the servo? Had I have known, I may have put a quick disconnect on my battery so that the original bad servo would not kill it.
Sorry I can't help much but Larry replied to my post a few days ago (Vacuum or what?) and suggested checking the fuses and go for a vacuum leak.
I am curious about the amplififer as a cause for all this.
I've listened for a vacuum leak with a stethoscope with no luck.
I too am waiting for a chassis manual (which is on back order) which will hopefully help trace the vacuum problem.
First one with the fix should get a prize