Check the fluid to make sure it is not antifreeze. If it is, the heater core is leaking and needs to be replaced.
The other likely item to check is the A/C drip hose. It drains condensatiton off the inner evaporator and onto the ground. Sometimes it gets criomped and backs up into the A/C pan and into the interior.
If it is not either of those, then there may be a problem with the fresh air box which is the cowl just under the windshield. It takes in fresh air for the A/C heater. There is the maion grate that you see from the out side and depending on the design, either the grate is removavble if it is plastic and heald in with palstic plugs or you have to work with ir under the dash if it is metal and part of the body. Essentially noth will have a few layers of cowling to direct rain away from the fresh air inlet which is right under the fresh air box. It is usually on the passenger side but can vary on some vehicles. The rain gets directed around the cowls and into two weep holes at either end of the box that drain out just over the rear of the front fender well. You can check the weep holes by pulling the inner fender well cover off and seeing if they are clogged. If you park outside all the time or have small debris around that can easily get past the size of the holes or slits in the grate, it can glog the weep holes and flood the box causing it to overflow into the interior.
The other posibility is that the windshield has a leak. They are glued in place and can leak especially if you have been in an accident.
The best way to start after you check to see if it is anti-freeze is to have someone help you one day when it is nice out. Have someone under the dash with a flashlight and someone with a garden hose outside. First put water into the cowl grate only and don't let it get onto the seam of the windshield where it seals to the cab. See if water starts to come through and also check if you can pin point the area where it is coming through.
Again, look for the A/C drain hose which is on the bottom of the firewall and should be a clear hose. Make sure it is not crimped and when you run the A/C on a humid day it will always create a puddle pretty quickly if it is working right. So if you don't see a puddle under the truck after a few minutes of full blast A/C, that may be the problem.
If the windshield appears to leak, take it to a glass shop and have them re-glue it. If you have had a windshield replaced before, you may be able to do it under warrantee.
Monday, January 13th, 2020 AT 8:20 PM