Thanks to Bob Brooks for the writeup
I had said I would post a few pics following my heater bypass install. It can't get much simpler. If you know how to remove the duct work and the heater motor, you have done 80% of the modification. I purchased this from Tom Fitzpatrick at http://vintagebus.com/rstube/ for around $200.
Hints: As this was made for more of a 964, here are a couple of tips to install on our cars.
1) In some of these other posts they tell you to purchase a new 964 part for the top of the fan. The TT does not need it, and in fact it would position the tube too far back with our intakes. (it only cost me $45 to find this out).
2) There is no need to make a box and stuff in some resistors. I just cut a nice hole on the side for the coil resistor, and a little slice in front for the sensor. You will need to plug the vacuum line that went to the sensor / pot on the front of the original duct work.
3) Plug the front metal hole on the tube that is used for the dual distributor hose. I just used some silicon rubber on the inside.
4) You will need to cut this new tube to fit a TT's heat duct system. Here is a picture of what I cut off the bottom. Careful with the angles and don't cut too much. You can always file more if you need too.
That's about it. The hose and clamps will work fine at the bottom of the tube and give a secure fit. Remember that the top clamp is squeezing the tube so do not overtighten.