It ended up being several (three) gaskets around various entries into the cab from the engine compartment through the firewall. The entire dash had to be removed. I replaced the heater coil since it was down that far.

 

I have a simaliar problem but only on driver side floor board in a 2001 what seals did you replace

 

Mine leaked in rain. Was at cab seam on driver side roof, had to peel off plastic cover strip and fill void behind rear window seal at seam with silicone. 5 min fix, took a while to find. Had to remove carpet to dry out truck, that took longer. Rain will leak in and drain down pillar then flow along floor board and pool in floor pan under floor mats. Also had leak in AC drain, housing cracked under dash on passenger side, pain to fix.

 

I replaced all of them. The main culprit was around the heater core. I replaced all gaskets and the heater core since I was there.