A few weeks back I ordered a tach converter from Dakota Digital (the DSL-1) and I finally got the chance to install it last night into my caddy.
I used a tach from my old rabbit GTI and it fit right into the cluster as it should. The "circuit board" (film) on back of the diesel cluster already had the right circuits on it and all I had to do was swap over the tach and put the washers and brass nuts on where they were supposed to be.
When I replaced my alternator last time, I got one with a W terminal, so all I had to do was run a wire into the cabin for that signal and then find a keyed power and ground. The tach signal goes up to pin #5 on the instrument's cluster's plug. It's a red wire with a black stripe and it was already in the harness despite the fact that the car never came with a tach. Pretty cool.
Now... since the tach is from a gas car, the redline on it was not exactly appropriate for the TD, so I took some markers and recolored it to make the "orange zone" start around 5k rpm. Seemed to work out really well. Right now the TD only revs to about 4300 before power falls off, but hopefully the Giles pump will help that.
Calibration was easy with the buttons on the DD adapter and since I knew from the dyno what my idle was (a little high at 925 or so), it was easy to set.
Here's how it turned out:

In the pic you can still see the center hole / post from where the old clock was. No way to get rid of that, but it's not really noticeable when you drive.
So far, the tach is working great. It's interesting to see what the actual rpm's are when I shift and on the highway. With the FF five speed, I'm shifting at around 3000 puttering around town, just short of the torque peak at 3400 on this engine. Interesting. The DD adapter works as advertised and it's nice to have a tach...