Rev counter...Why??

1989 golf gti cabrio.
Firstly my rev counter does not work. I can live without it. But !! After not starting the car for 3 weeks the battery got weak and as I tried to start it the rev counter went up and down as it barely turned over…It went half way then as the battery died it went all the way up.. What is going on with the rev counter doing that when it does not work??? any ideas please?

Bad grounds and a iffy 10V stabilizer in the instrument cluster as well as possible iffy connections to it.  

I would try the following first.
Plump the connector on the Instrument cluster.
make sure that you have a good 14V+ (battery) on the black wire, and splice a ground from the Frame to the Brown wire (pin2 usually) on the connector side.
Take all the Brass Screw/nut off one at a time and polish them shiny with steel wool and replace, some time the screws and nut work loose.

The Coil is usually the Drive wire via the Fuel pump relay for the Tach so you want to be sure that the wires on the Coil are tight and clean as well.

See my how-to''s in the archive section.

The Tach moving is a good sign that it is partially working.

The Test for the 10V stabilizer is that there is Battery on one leg (12.8-13.8v) to the middle leg (key in and in the run position Cluster attached, and then from the middle leg (ground) to the other leg is 9.5-10V.

I would also test from pin 14 (black) to the middle leg that you are seeing 14V there and from pin 2 to the battery leg.  As the mylars have been known to break and delaminate.  I also have a how to back feed the cluster if it is missing the B+ at the one leg of the stabilizer.

My rev counter did the same when the battery was going bad.
Car ran but needle on revs only moved off the bottom when rev'd high and was probably about 2000rpm behind the engine.

When the battery died I tried to charge it but it didnt even register on the charger

New battery and everything was fine but eliminate it first with Briano's tests and checks