I believe you need item 10, VAG Part N 903 653 02 in the Etka illustration below, presuming the leak is coming from item 9, 06B 121 065 AC, the coolant return pipe. I infer that item 10 is a 20x3mm "round seal", but it's not apparent exactly what it looks like. (click to enlarge the illustrations)

FYI, item 2 is the thermostat (which refers you to page 121-10).

You can search for the part numbers on ECS Tuning, for the pipe at least, for good pictures.

 

I name it Bob. I can't believe no one else named it

Yes, the 20x3 O-ring is what it needed. It's held in place by bolt #11 listed above. I expect it's far easier to remove the pipe and change the O-ring with the manifold out of the way. It's also a great time to overhaul the PCV, so I would add the intake manifold gasket and PCV to your parts list.

 

I replaced mine with Dorman 902-991 which came with the o-ring. It was $10 at Autozone. I left a review of the installation hiccups I encountered. You can't beat $10 with the o-ring. "Bob" was a bit of a PITA to replace but I did it without removing the manifold which made it harder so that may be why.

 

That's for the thermostat housing, not the metal pipe that going into the back of the water pump from the heater core.

 

Oops I had skimmed the first post as thermostat housing and not next to thermostat housing.

 

Is the EKTA part number for the lower (return) coolant pipe the same for the AUG engine, i.e., 06B 121 065 AC?

 

Yes. The AUG is an 06A block like the AWM. AEB/ATW were 058 blocks.