The easiest way to find it is squirt some in and see where it comes out at the bottom. I've found that if I undo the top door hinges I can get the flexible pipe of my underseal gun in and a good way across the bulkhead under the windscreen inside some sort of box section. While you're in that area, take the door seals off and have a look at that pinch welded lip that they fit over - mine's apt to go rusty if left untreated so put a bit of Dinitrol on it every couple of years and it doesn't seem to be deteriorating further. Also, there are some holes along the bottom of the door. I squirt some up there every couple of years too. The probe fits up my holes (said he without a trace of innuendo) but on my friend's I had to ream them out with a 6mm drill. Well worth doing, because when I have the door trim panel off, the door frame is nice and black looking with no active rust.