The Trane VS airhandler has a lip that allows the supply duct work to get taped to. What you need to do is get some of that duct board used to fabricate duct boxes and ducts. Cut it large enough to fit around the joint to the top of the airhandler. Cut it long enough and just make cuts on the inside insulation at the point that will have bends. This allows you to use one pices and bend it around the joint. Once you get it fully around the joint, use metal tape and secure it. The width I used was wide enough to go from the top of the airhandler to the ceiling. In my case this was only about 3 to 4 inches, since my AH sits on a pretty large return box and EAC.
I had the same problem, my Trane VS airhandler is also in the garage. What happens is the condensation builds at the top, puddles and then finally runs down the unit. This fixed the problem.
BTW I have heat strip heater so the block off was not needed. I noticed the problem more this year than the last 2. Humidty was higher.

[Edited by kevinmac on 10-13-2005 at 09:38 AM]