Does the back of the cab wall get wet? That would be a third brake light.
You may need to lay towels down everywhere. On the dash, all overt eh floor, on the door, on the back wall, and even on the sun visor. Hopefully the towels will point you into the right direction.
You could also dump a couple gallons on the roof and let them run down the windshield to the cowl. After the water runs off so you can open the door. Look inside and see if its dripping or running down anywhere. It maybe easier to pull the carpet for this.
99 x-cab 4x4 4.0l auto
mods, cranked t-bars, silencer delete, k&n drop in, Rebuilt trans ,Trans Go stage 3 shift kit, 3200stall Fuddle Racing Torque Converter,
VANIR 63mm TB, Roush split rear exit exhuast, E-Fan, JBA Headers
Auto up/down both windows, OHC, EATC (soon to be modded)
03 Crown Victoria Police Interceptor