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A/C radiator fan circuit/relay

I'm having a small electrical problem with my '83 Ur Quattro. The radiator
cooling fan is not switching on with the A/C. I tracked down a related
problem with the main cooling fan relay - a whole bank of relay coils had
no ground! [So what the hell are all these damn relays if the car runs
fine without them?]. My Bently manual has a '84 wiring diagram that seems
to fairly closely match the '83 models. It shows two "RAD FAN THRMO SW"
controlling relays driving the fan via two separate windings in the fan
(??) - there is one ground out of the fan and two leads into the fan from
the two +12 relay sources. I can't find anything to identify the "second"
thermoswitch, so I presume it is in fact a mechanical switch/relay that
is activated by the A/C switch. I can't find the A/C relay -- can anyone
point me in the right direction? The '83 fuse/relay panel is definitely
different from all the ones in the 84/on Bently manual (only has 15 fuses,
not 20-odd, etc.) - and the main radiator fan relay was in another bank
of relays hanging under the top of the dash just to the left of the steering
rack. None of the wiring seemed to match the color codes the book claims
for the '84 A/C relay (it did match the main radiator fan relay, thats how
I found it).

Any help gratefully appreciated!