Sorry, the 64s did not have a drain tube. Most of the time, the condensation just ran down the outside of the trailer.
I recently put one in mine. http://www.airforums.com/forums/f427...ks-108118.html
I have same year and model as you, though not all were rear twins.
The solution I came up with is working just great. The part that was almost unbearable was drilling through the side channels to get the drain tube through the roof and down into the overhead storage area. You really want to think it all through and make sure you don't dent, or especially puncture the skin with the drill bit if you go this way. I saw one guy just drop it into the coach area so you could see the drain tube, but I couldn't bear that option. The only way to truly do it right is to remove the interior rivets and skin and snake it down, but I didn't want to do that either.