I used an Autometer Tach Adapter (part #9117) to get a good tach signal on my MKV GTI.
First you'll need to locate the main power supply to the coils. It's located through the loom behind the coils and the best way to get to it is by removing the plastic elbow. At this point you may want to remove your intake (or simply spin it out of the way) to make it easier to access this wire. I didn't do this initially but about halfway through the install I decided that it would help:
Separate the red wire from the rest of the bundle. This is the main power supply to the coils. Strip back some of the factory shrouding to reveal more wire and give you some slack to work with.
Now you need to mount the Tach Adapter box itself. Autometer recommends mounting it on the firewall but in this particular application there isn't sufficient room between the intake and the firewall. I looked for another suitable place but options are limited. I ultimately decided upon mounting it to the fuse box cover since it's one of the only flat surfaces available within a short distance from the wire we're going to tie into.
The tach adapter has 4 wires attached to it; a black wire that gets tied into chassis ground, a red wire that goes into the upstream half of the red coil power supply wire, a red and gray wire that goes into the other half, and a gray wire that supplies the 12-volt RPM signal to the secondary tach, shift light, or data acquisition system.
The red and red-and-gray wires attached to the tach adapter aren't long enough so I added about another foot of 16 guage wire:
After soldering and shrink tubing the connections I routed the wire along the firwall towards the coil supply wire:
I then soldered the red and red-and-gray leads into the power supply wire:
Next I needed to rout the gray wire from the tach adapter into the cabin. Remove the battery and the battery tray to allow you access to the grommet in the firewall that you can use to gain access to the driver's side footwell:
Cut one of the nipples on the grommet and pass the wire through the hole and into the driver's footwell. From here I attached the 16-pin TraqTach connector that supplies the 12v signal to the Traqmate Sensor Unit mounted under the dash.
Set the Traqmate to record RPM and set the number of cylinders to 4. Go test the unit and verify that it is reading correctly.
You can also set the Traqmate up to calculate which gear you are in. In my case I have a DSG transmission which has two final drive ratios (one each for gears 1-4 and 5-6).
Gear 1-4: 3.94
Gear 5-6: 3.09
Since the software only allows one differential ratio to be entered, I had to change one of the other variables in order to arrive at the proper wheel ratio. Tire circumference was out of the question so that left me with entering pseudo-trans ratios for gears 5 and 6. I multiplied the actual diff ratio for gears 5 and 6 (3.09) by the trans ratio (1.090 and 0.920 respectively) to get the actual wheel ratios for gears 5 and 6 (3.370 and 2.840 respectively). Then I just divided these wheel ratios by the diff ratio for gears 1-4 (3.94) to get the new "fake" gear ratios for gears 5 and 6 (0.855 and 0.722 respectively).
Here is a screen capture of the software with the ratios entered: