You could always try a vacuum on the drain and see if it pulls out whatever is blocking it. And don't forget to use the line now and then "now that I'm retired I NEVER get a day off"

 

Both my 96 and the 2000 I no longer have, leaked condesate into the cab. The piece of hose fixed both of mine. I think its 5/16 OD hose that fits into the pee hole. Last week the 96 leaked again in the cab. I checked it out and found a little debris in the end of the hose that was in the pee hole. When I pulled it out, it ran about a pint or more of water with a big ole stream.

My son has had the 96 for 10 years. He more less gave it back to me. I'm chasing some odds and ends again. Best running truck ever built.

Dave

 

I finally got around to installing a rubber hose on the drain and ran it down to the frame and secured it with wire ties and it hasn't leaked since. Sometimes I have to prove how hard headed I am before I do the simple fix.
I am keeping a close watch but I believe the problem is fixed. I need to do some reading and learn how to add pictures etc. so I can post a picture of the installed tube for others benefit. I also have other things to work on and would like to document them also.