There is not really a specific way to clear it. I would jack up the car then get under it with a thin piece of metal with a small hook on it and stick it up there and try to pull out whatever is clogging it. I say PULL OUT because you don't just want to jam the metal rod up there and push the clog into the ac condenser box. Hopefully you can kindof finaggle the clog out of there. Other than that, you're looking at removing the entire dash board, dash board support bar, and the heater core to actually be able to remove the ac condenser box to completely clear it.
compressed air is another thing to try, but I think that will have the same results and just push the clog back up in there instead of pulling it out. And as far as I can remember, that plastic drain thing jammed into the firewall doesn't just pull out. But it might so I would try to just pop that out if possible.
the real question here though is how did it get clogged in the first place? debris can't get up in there from underneath the car and only water drains out which wouldn't clog it. UNLESS the ac condenser box is actually cracked, which is possible because a passenger could kick there foot up under the dashboard and potentially crack the plastic box that holds the condenser.
Nah...not that simple. Crap from the outside slowly builds up in the evaporator box (not condenser), and eventually blocks the evaporator and the drain tube as well. Assuming you got a 2nd gen, with no cabin air filters, you can take off the glove box, unscrew the 4 screws holding the two halves of the evap box together, and carefully pry it open a bit (don't crack it). It'll just be enough for you to get in there with a hook or something, or as I did it with a 1/2" vinyl tubing duct taped to a vacuum hose. You can also cut out a square opening on the right lower side of the evap box with a sharp knife, and use that hole to clean it out and then duct tape the piece back in.
Forgot to mention one thing....Unless you have a pool inside your evaporator, most likely you have a crack in the evaporator housing. I don't think there are any seams other than the hole for the pipe on the lower half. So if it leaks, someone probably kicked it... You'll have to check!
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