This was a 4 part problem on my truck.
1) There are 3 main passages through the firewall to the cab that were leaking on my truck.
- Heater core passage
- AC line passage (condensation can and will drip down the firewall and into the cab) - i'm still considering an insulating wrap
- AC drain line passage.
I sealed all 3 with silicone and foam.
2) My heater core was completely clogged (along with the rest of my cooling system) and did leak into the cab
3) The drain line at the bottom of the firewall is letting water drain back into the cab. Get a 3-4 inch piece of 5/8 heater hose from autozone and a hose clamp and put it on the drain line. This will prevent water from flowing out, running along the bottom of the drain outlet tube, and back towards the firewall.
4) My 3rd tail light was leaking. $3 roll of foam from lowes and that was fixed.