By strange coincidence, I am building an Atmel microcontroller based
interface board to make this 92 Passat gauge cluster work with my van, and
I'm working on building just such a warning system into it. My solution is
probably overkill though. Mike S's quick n' dirty schematic is mostly all
you need... except for the coolant level sensor, which I think is a bit
touchier than a simple "got ground" OR "got 12v" = "start buzzing" system.
Since the oil pressure system has its own buzzer, I reckon only the coolant
level, coolant temp, and alternator need audible warning systems added.
Alternator is easy. Blue wire turns to a ground when the alternator stops,
so that's easy. Coolant level is tougher. It goes open on fault, and is in
who knows WHAT state when it's OK (probably some random resistance to GND).
Coolant temperature isn't much easier a variable resistance to ground, but
documented in the Bentley. A buzzer on the blue wire would be good for
catching a thrown alternator/water pump belt. Don't know what to do about a
stuck thermostat (coolant temp warning) or a coolant leak. Those are pretty
easy to do via software with a microcontroller, but probably a little wormy
for simple transistor or relay logic without spooking the existing
instrumentation. Probably the only "easy" way would be to scab on to the
existing warning LED to drive a transistor... and even that gets into
Soldering Iron territory.

--
John Bange