air conditioner leak at firewall

All air conditioning units' evaporator coils (the one that gets cold) get wet from water vapor condensing on them, like a cold beer on a hot day. The more humid the air, the more condensation there will be, and it will need to drain. I don't know the Dakota specifically, but I will bet there is a condensation drain below your evaporator coil that is clogged. It will be a tube that is supposed to drain underneath the vehicle. If you can find it from either underneath the vehicle or under the dash, clean it out with a piece of wire (carefully!) or other such implement and all should be well. Question...after you have driven awhile, and you park your truck, is there ever a puddle of water under it? If so, the drain is only partially clogged, and you can find the drain by looking directly above the puddle. If not, it will be more difficult to find, but not impossible. Good luck!