Here's how to fix the VW tach for use with tach signal that comes directly from the Subaru ECU.
Starting with the Tach Schematic and the following picture of the tach PCB:
1.) Remove the resistor in location 8, and put it in place of the resistor in location 14.
2.) Remove the resistor in location 10.
3.) Remove the diode in location 9, and replace it with a piece of solid wire, bus bar, etc.
4.) Replace the resistor in location 13 depending on the RPM range you want for your tach:
6000 RPM = 8K ohm
7000 RPM = 9K ohm
8000 RPM = 10K ohm
Note: By making this change you will need to re-calibrate your tach. This requires applying a 12V square wave of a known frequency (i.e. 133.3Hz for 4000 RPM) and adjusting the potentiometer in location 3. Since the tach circuit isn't perfectly linear, I find calibration at 4000RPM a good choice. This makes readings from 3K to 5K dead on, while readings from 1K to 3K are about 100 RPM high and readings from 5K to 8K are about 100 RPM low. If you are not sure how to do this calibration, or do not have the equipment to do this, it is probably best not to use this tach fix.
Your PCB should now look like this (except resistor 13 should be 10K for an 8K tach):
Using an 8K tach face plate from a GTI (year?)