Thanks for all the responses.

No Cabin Air Filters (wish I had).

Where exactly is the "Klima Relay" and how would I check it?

I don't know the exact vent temperature but it was plenty cold when the system was working. I'm now back in Canada so A/C won't even be needed for a few months, but I want to get it fixed before I head south again in May.

I was thinking that perhaps the ACC/CCU might be the culprit, although it seems to work on all functions otherwise. It probably wouldn't hurt to pull it out and go through the re-solder process anyway as I'm sure it hasn't been touched before.