The evaporator drain is very low on the passenger side of the firewall, just to the left of where the firewall shape changes for the transmission tunnel. Its small and impossible/difficult to see from above, looking down into the engine bay, so best to jack up the passenger front and slide under the truck and look up. Its pretty small, maybe half an inch of black rubber. Kind of just protrudes from the firewall. Nub-like, don't expect to see a long hose.
Its been mentioned on this forum before that some people have been able to pull the inner fender liner away at the front passenger wheel well to access the drain, but I can't speak to experience on doing this.
Otherwise, that must be one hellova leak around your sunroof to A) let that much water in, and B) to also let in so much gunk that the drains are getting clogged..