Location:
A-pillar  is elegant if you can get the Autometer pod, but it seems to be permanently out of stock. part # 20691 for one- gauge pod, or # 20690 for double pod. Originally designed for 1993-97 supra, but heat gun will  make it fit, according to many previous posts. Next choice was the  left-center vent, prior posters show nice looking pictures.
Loss  of air cooling? If you turn off the vent, you will feel an increase in air from the other vents, so I concluded that total air loss will be negligible if you use the vent for the gauge and lose the vent

Which gauge :
Autometer # 3401, Sport-Comp style, 2 5/8", looks  a good match to original TT gauges. About $50. I put a red condom on the light, not sure I needed to

Installation technique:
1:  Remove vent:  ( the circular alum. ring and  internal black plastic air diverter and valve.) : just slowly pull out with your fingernails. Once out, separate the alum trim ring, which will be reused,  from all the black plastic.  this can be done with nothing breaking, just pull slowly and carefully, it will come apart.

2: Cover the  now-useless air supply tube.  Visible inside the dash, thru the hole :
I cut a piece of corrugated cardboard a little bigger than the tube, punched a hole in the cardboard, tied a piece of string  through the hole, (so I could retrieve it if it went in too far.) Then,  just push the cardboard  slowly in, forcing it a little, to make a very tight fit, and this  mostly sealed off the air. For good measure, I put some caulking on the  joint between  the card board and the air tube, both for  better sealing and better adhesion, so the cardboard would not  be pushed out by the air

Physical installation of gauge:
3: The inside of the  now-open hole in the dash is a little too  tight for the autometer to fit, but a few small saw or knife cuts will open it up a little. Keep in mind that the friction between the gauge and the inside of the hole is all that holds up your gauge, so don't cut too much.  Keep trying until you have just enough removed to leave plenty of friction.  After hooking up wires and tube, You need to get the gauge in the hole and  somehow hold the TT alum trim ring on. Although the autometer gauge fits right into the alum ring as if it were designed that way, it visibly looks great but there is nothing to hold it there. I put some clear caulk between the alum ring and the gauge, and pushed gauge and alum ring into place, then held the ring centered on the gauge with scotch tape while the  caulk dried.  The friction holds the gauge, and the caulk holds the alum ring on the gauge.

Wiring:
4: The light can be attached to the output of the  dash light dimmer switch, the blue-gray wire coming out of the  dimmer is the output. Just scrape off some insulation  wrap your supply wire, solder, tape. For a ground you can  use the factory supplied ground post that conveniently has  room for other wires, (it's a 10mm nut, on a stud that faces to the
rear of the car  under the dash)  Brown is the  ground color for all VAG cars, so  you can confirm you have the right stud  since only brown wires will be there, and it's visibly connected to  unpainted sheet metal. The  nut was surprisingly tight, I bent the support before the nut would budge,

Air hose:
5: VAG  very politely put a 1/2 inch grommet above the accelerator pedal, that  opens to just above the brake booster, perfect for our purposes.  Push the grommet out, punch a hole, and thread the air hose through it. Where to connect air hose to intake vacuum? There are a variety of options, I think the easiest is the  vac hose that goes to the fuel
pressure regulator, at the drivers side of the pressure rail. (other options include the diverter valve) Cut the  hose  about an inch from the FPR, where it makes a sharp turn down, and insert a 3/16 tee. The tee fits in snugly,  but I put some probably unnecessary silicone glue  on it. Get a 3-6 inch piece of the same 3/16 tubing,  put the 3rd opening of the tee into one end of this new hose piece. The air tube just perfectly fits inside, push it in a few inches to the other end of the new hose, and again some silicone for  air tightness.

Lower dash remove and install( required for access).
Removal:

1:  2 Phillips  head screws that hold the  hood  release  knob,  leave knob laying loose
2:  all screws from now on are torx-25 ,  interchangeable, identical
3:  2 screws  at  bottom of lower dash, near  drivers seat
4:   fuse box screws; need to remove all, so that the large flat plastic fuse-holder with  fuses is  loose (it is possible to remove dash with  only some fuse box screws out, but you'll never get it back without all screws out and plastic piece loose
5:  Unclip 3 wire connectors: one at the obd port, and 2 at the light. Just squeeze  the 2 plastic clips and each one comes right out. Make sure you  find the right place to squeeze, no significant force is requires if  squeeze right.

Reinstall: This is actually the most difficult part. Don't forget to reinstall wires first.

1:  need to  push lower dash piece towards front of car using horizontal movement, not upward, since the  hidden, front-protruding part of the light switch fits on top of the fuse holder, and can only enter its space with horizontal approach.
2:  There are the  2 white plastic holders near the firewall, also require a horizontal approach. But at least the plastic of the lower dash is flexible enough to allow some forgiveness.
3:  2 metal funny-web-shaped  pieces fit into matching slots in upper edge of the lower dash.
4: Line up the  left edge of the dash so it  goes inside of the loosened fuse panel
5:  last step: If all is lined up properly, 2 plastic captive  clips on the lower dash snap-fit into holders on  upper dash

Takes about 4 hands to do all this, and of course no room for more than one person to fit, anyway( you are lying on your back, head under dash looking up, feet in the air.

Next install all the torx screws  Save the 2 Phillips for the hood release knob.