Since I do not know the technicals of your rig, I can tell you sorta where to look.
Look on your firewall, maybe low, get an idea of where the bottom of the heater box is. Look at same height on the firewall. Probably center or towards the passenger side.
I did not say this would be real easy.
Maybe look for some (or not), dampness around a rubber apparatus or rubber tube protruding from the firewall. When you locate it, you can do one of the following: Poke something in it, make water flow out. Sometimes this a bad Idea.
"Why?", You say
Been there done that.
If you poke it, you shove the blockage back in, you will be doing this "unstopping thing" over and over.
Dependent upon the accessibility to the drain in a tight spot, you might have to obtain and bend some tubing (copper ice maker line, aluminum, plastic etc) and do some "fishing" to get it to go into the cars condensate drain tube. Just need to get into the end, maybe 1/4 to 1/2 inch.
Once this task is achieved (or figured out), rig the tubing and or combo with rubber hose, into the hose of your "shop-vacuum" Using duct tape or other, seal it all together this is just a field expedient reducer from the vacuum hose to the tubing, so that the vacuum efficiently can suck the hose/tubing with no vacuum (suction) leaks.
(See my pictures below from working on my mothers 1986 Thunderbird's drain)
You already know what to do next.
Suction out the drain tube on the car. So that you draw the stoppage into the vacuum. Along with all of the water. And any other potential. Later on stoppages. (In other words, once it starts draining on its own. Do not let it. Suck it until empty!)The "rush" of the water may draw out other stuff in the box.
Keep in mind, we are not performing heater box arthroscopic surgery, do not shove it (hose or tube)past the firewall, you may tear or disconnect the car's drain. Then the drain nozzle may leak between the firewall and the A/C box inside.
Sometimes there is a lot of water so expect it!
Please let me know if my method worked for you, and did I explain the procedure well enough?
Images (Click to enlarge)
Monday, November 10th, 2014 AT 5:52 PM