I put a generic PCV elbow on my A/C condensate drain... it stops the condensate tracking back on the underside of the nub that comes through the firewall, past the decade seal and down the inside of the firewall under the carpet, to pool on the pax floor pan... when you accelerate, it surges along the inside edge of the sill and into the back corner of the cab, making it look like a rear window leak.
The rear windows and the 3rd brake light are a problem, but I'd bet on the A/C first.

From the pax floor, look up at the underside of the climate control box (A/C-heater)... to the left hand end push the carpet towards the firewall, and you should see where the drain goes through the firewall... tuck your finger through the opening in the carpet and bend it down to feel the back of the padding behind... is it wet?... you have your answer.
Look for the stub on the outside of the firewall and render your fix... this could be some tube to lead it away, or as simple as a medium size tie-wrap around the stub... as the water tracks along the underside of the tube, it will hit the tie-wrap and run down the edge and drip away, rather that continuing inside (well, that's the theory).
I used one of these;


As for the window; I sealed the end of the track on the inside, as the stops had shrunk away and left a gap... this and clean the weep-hole on the outside.