I just had this problem on the weekend, allthough the freezing water was only leaking onto my girlfriends feet! I suspect my drainhole is blocked or something. I will check it out one day, but it was leaking between the evaporator box and fan speed controller...
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)