If you have, condensate forms on your evaporator (sorta a radiator looking A/C thingee in your heater box!)
Eventually, it forms bigger water drops, and falls into the bottom of your heater box.
The water can only escape one way. The CONDENSATE DRAIN!
IT MAY BE STOPPED UP!
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 didn't say this would be real EZ!
Maybe look for some (or not), dampness around a rubber apparatus or rubber tube. When you locate it, 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'll be doing this "unstopping thing" over and over.
Dependent upon the accessibility to the drain in a tight spot, you might have to bend the tubing and do some "Fishing" to get it to go into the 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-Vac"; Using Duct Tape or Other, seal it all together----this is just a field expedient reducer from the vac hose to the tubing, so that the vac efficiently can suck the hose/ tubing with no vacuum (suction) leaks
You already know what to do next. DO IT!
Suction out the drain tube on the car. So that you draw the stoppage into the vac. 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 crap 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 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 (OR NOT), and did I explain the procedure well enough?