89 200tq wagon

["Thomas J. Donohue, Jr." <donohue@netconnx.net>]

I'm new to the list and am amazed at the volume of traffic that arrives
at my email doorstep!  I have an 89 200 TQ Avant with 344.5k miles (not
km!).  The red brake warning light has been either on or, occasionally,
off for about the last six months.  There is nothing wrong with the
braking system per se (all fluids are normal, just replaced all pads and
rear rotors, etc.).  Anybody help with location of the sending unit that
must be giving me this problem.  I have had several experienced Audi
people (I'm definitely not a mechanic) check it out and all, including
one guy who's a mechanic at the local Audi dealer as his day job and who
moonlights on his own, can't seem to figure out the problem.  Any ideas
I might be able to pass along to the wrench spinners will be
appreciated, especially by me.  By the way, I do have the car for sale
if you know anyone interested...it is tight, solid and I'll be happy to
give interested parties further info.  Email me with solutions or
interest at donohue@netconnx.net.  Thanks