Fairly new LR3 owner here. In an effort to be exhaustive in my search I am trying to look for signs of water leakage and wiring problem points.

I've pulled up the carpet in the front two corners and all looks dry, but I'm wonder how far back to disassemble before you're reasonably certain that water has not made it's way down there.

Also for the windshield frame, what steps do I take to check and then eliminate possible water entry points?

 

I'd say if your carpet is dry then you're good. It's fairly obvious otherwise. For the windshield cowl, here is what I'd suggest:

1. If it's warped, get a new one and remove the old one.
2. While the old one is off, create a shield and attach it to the pollen filter box just below there. (I used a plastic license plate "insert" from the dealership)
3. Put the new cowl on, and put a bead of RVT down on the windscreen/cowl interface.