Hi as far as I know it seems that the amp does affect the AC. But maybe not.

I had the amp replaced many years ago when my heat went out and besides getting heat back, the AC began to work again.

BUT, recently I had the amp removed because the servo got stuck ON and it was draining the battery.
The AC works (my ac is not extreme cold anyway-but I can feel the difference btw air and AC) and I hear/feel the compressor kick in when it is switched on.

check your fuses for breaks and weak ones.
check your blower fuse. if it is weak or burnt maybe the blower has too much resistance due to age....
good luck

re:'79 300D..same servo etc.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD