Beetle---- At ECM under driver's side top dash panel (Opposite of where the cabin air filter lives). Its on the left plug, light green/reddish stripe.
Jetta----- At ECM underneath the valence that's under the windshield wipers. If you look through the slots in the plastic, you'll see it, dead centre. Pull off wiper arms, remove panel very very carefully. Tach wire is on the right plug as you're looking at it. Light green/orange stripe.
Of course, you will want to double check that these are tach wires with a proper tach probe that gives a clnder/rpm count. A DMM set to AC will not give accurate information. With a DMM, any multiplexed data line will be giving of the same approx. amount of AC voltage as the tach line. One other thing.... you won't get tach at the cluster that is very usable. There's other stuff on the line, and it isn't a clean signal.
Hope this helps..... how'd ya like bypassing the Immobilizer /images/graemlins/smirk.gif Or'd you go the route of using the factory antenna and an interupy relay?
Good luck... I do this for a living, and the VW stuff is the most interesting by far... I average 4hrs on an automatic Jetta.... most of the time is spent getting wire into the door(alarm disarm, keyless, trunk/hatch, fuel door if so inclined)
Gus