Had my heater core replaced by the dealer 2 years ago. Now getting fog/condensation from the vents and defroster. Dealer wants to replace it with a "better" one for $800 more bucks. I said hell no!,F off and what every other sane person would tell those losers. anyway, anyone know what causes this? Could it be a hose or clamp in there loose or maybe something else. I have seen no fluid inside but my oil changing folks say they saw fluid leaking outside but not confirmed.

Thanks

Sounds like a pin-hole leak in your 2 year old heater core.

Ford had a problem with heater cores being destroyed by electrolysis. The coolant was carrying low voltage which was eating the heater cores rather quickly, dealer could check your coolant for voltage, it was very small amount,, not even sure how it happens, maybe poor engine ground or something like that I can't remember where I read this, do a search maybe it'll come up..

Got the info right here, not sure if it's your problem, but:

http://autorepair.about.com/library/a/1i/bl486i.htm