'89 100 Thermo-switch and Compressor questions

[Adam and Soma Dixon <biwltud@bellsouth.net>]

I recently bought an '89 100 and attempted to replace the radiator
thermo-switch, thermostat and lower hose.  Upon reassembly and the
addition of coolant, a leak occurred at the thermo-switch.  The new
thermo-switch had a gasket with it, which had a felt-like surface.  Was
this really the right gasket to use?  Should I have used some teflon
tape on the threads?  Based on the size of the leak, I really goofed and
would appreciate being set straight.  

Also, the A/C compressor isn't working yet.  When I removed the
radiator, I noticed a green wire running to the compressor, which has an
intermediate pair of connectors placed almost under the radiator (ie.
out of sight!).  What is the function of this green wire?  Could it be
the reason that the compressor wasn't kicking in when the Climate
Control unit was set for LO?  I have looked at the Haynes '84-'88 5K
manual (from what I am reading, it seems that Bentley is the only manual
choice for my car) and haven't come across this wire in the schematic
set.  

Also, are there any diagnostic tests that I could perform on the
compressor?  I have checked the A/C fuse and A/C relay.  The relay
closes w/12V applied, the internal capacitor is approx. 570uF and the
diode is showing a .6V junction drop - so I am assuming that it is okay. 
Thanks!

Adam Dixon
'89 100 (first Audi experience!)
biwltud@bellsouth.net