I have recently (last couple weeks) installed a tach in my 1982 Westfalia
diesel-> 1986 Jetta GLI conversion.  Did surgery on the console to remove
the clock/gas gauge/temp gauge, and replaced it with the tach/gauge module
from the Jetta Console.  I did long painful circuit board traces and study
of the wiring diagram from the Jetta Shop Manual to figure out where
everything should be connected, and then patched over the connections for
gauges and tach to the old diesel circuit board.  Since the diesel had a
line for the glow plug indicator that ran forward in the wiring harness to
the console (now unused) that was the connection to the coil contact point.
The back side of the console looks like it has had some surgery, but on the
front, it looks like a factory installed unit, except for the hole in the
plastic window where the clock set knob used to emerge.  It works very
nicely, so far I'm really pleased with the result.

Cheers,

Ted Hoekman
Dr. Theodore B. Hoekman
Faculty of Medicine - Basic Science
Memorial University of Newfoundland
St. John's, Newfoundland Canada A1B 3V6
http://www.ucs.mun.ca/~thoekman  (709)737-6703
http://cme.med.mun.ca