Hollow Door and Connecting Vertices

So I've been working with a new car model, going through and fixing some little bugs as I find them (L1s, L2s, Collisions, etc).

I've come across something I've not had to deal with before and after about a week of trying to fix it, I'm running out of patience and looking for thoughts on a better way.

The doors on this car are "hollow", there is an outside panel and in inside panel, but they are not connected to each other so the door has nothing in the middle and you can see right through it. It's very distracting getting in and out of the car in game.

I've done some research and read tutorials about this type of problem and everything says to go to vertex mode, highlight the vertices, and use the "connect" tool to automatically connect the open edges; but that hasn't worked well. So, I've been going through manually and selecting small groups of vertices, sometimes it works sometimes it doesn't.

I've made it most of the way around the door, front, bottom, and part of the back, but I'm at a point where it's not working anymore. No matter which vertices I select, I'm just cycling between connecting weird ones that aren't even selected, and getting the "Unused vertices remain" or "Cannot detect open edges" error messages.

I'm banging my head against a wall and I feel like there has to be a better way or something I'm doing wrong.

Has anyone had to fix this problem before that can offer any tips or suggestions? I shudder at the thought of having to do this on the 3 remaining doors when I can't even get 1 fixed...