to add a bit of closure to this strange thread. Turns out pin 8 of the amp had voltage, but when under load it fell off completely. I ran another 12V switched wire to that and tapped into that wire to supply it with voltage and presto, everything works perfectly, even my new(used) aux water pump.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg