Hey dude... I did this. Very straight forward, but there are a couple things you need to do.
Pm me and I'll write you up a bit of a DIY
Unless it's a secret, post it here for those dying to know.
Wasn't aware that this was such a wanted DIY... so I will divulge what I pm'd
Hey dude.
While I will agree that a bentley would be awesome... I didn't have the luxury of having one so I just figured everything out by taking apart the plug for the mk 1 cluster and reading the pinout (the little blue plastic wire dealy... it says what each wire is supposed to be.)
I used CE2 cluster, so one plug. It really doesn't matter because your going to need the pigtails from either of them.
A2 resource is awesome for telling you what each wire on the pig tail is.
The down side of how I did it, Ive got an early jetta... So no warning switch in the coolant expansion tank so the low coolant light flashes all the time. Im almost certain grounding the wire from the cluster would solve this problem, but it just doesnt bother me enough to rip the dash out again.
The other downside is there is an extra oil pressure/temp wire that I didnt have a place for, so it flashes as well. It also is attached to a stupid buzzer inside the cluster.
Now, you have two options at this point. You can ground both wires and see what happens. No guarantees wether it will work, but hey! Im no electrical engineer.
Option two, if you've got the low coolant sensor in the expansion tank, awesome. And if you can figure out where the oil light wire goes then bam. I personally took the cluster apart and disabled the buzzer.
Alright... your almost there. If you've managed to figure out the wiring and such thus far, well your still not done. You're going to need a speedometer cable from a manual mk2 car. Its the same length and everything, but different end at the cluster. Easy enough to do, just another step.
The mk2 cluster doesn't just bolt in either unfortunately. There is some trimming of the cluster to be done. That is up to you though. You've gotta just put it in, see where it hits, trim, try again. Mine is held in by one screw on the right side and then the rest of the dash holds it in place.
Now if I were to do it again, I would swap a whole mk2 dash in. To me it seems like alot less hassle, plus the cluster fits. The down side to this for anyone scared of alot of wiring is that it you would then have to replace all the switches and such to mk2.
Oh it goes without saying, but you need a diesel cluster. I crimped and shrink tubed all of my connections as well. But thats just me.
Good luck, its worth having a tach and an updated look. If you've got more questions dont be afraid to ask!
Old dash-

New dash-

Cluster-
http://s147.photobucket.com/albums/r317/casman66/?action=view¤t=IMG_0037.jpg&newest=1I don't have pictures of what I had to trim on the cluster, but its pretty straight forward. Trim a little, try again.
