OK - Here is a summary of what I found out about hooking up a tach to a VW A1 diesel (or really any diesel w/ the right alternator).
I got a VDO 6000rpm tach, part #333 054. This tach needs a signal from the "W" terminal on the alternator. The "W" terminal is usually unused, and is not grouped with the rest of the terminals, although it is on the back of the alternator. It would be rare to find a diesel VW w/o this connector (I am told). Anyways, once you hook that up, the tach also needs, 12V + & -, and you can also supply the same to it's light if you want it illuminated when the headlights are on. It does seem that these vdo tach need to be calibrated - mine seems to run @ 1000 rpm at low idle, which seems high. Since I don't have a known good tach to hookup, I don't know if the tach or my idle speed needs to be adjusted. If you found someone w/ a diesel tach, the vdo unit can be adjusted easily by turning one screw.
Oh yeah, you probably also want to get VDO's part # 240 101 mounting cup - the tach fits right in.
- Mac