When I said not to make it go outside, I meant not to make it poke through the visible, painted part of the car.

If you have a good connection point which happens to poke through into a wheel well or something, and you can reach it from the outside, then that's fine. Make it a full-on two-part bolt with a big old bolt head. (Head of bolt on outside of vehicle, thread and nut inside vehicle.) Scrape all the rust away, drill the hole to the proper size, and bolt away. Have a friend hold the outside wrench if necessary. When you're done, RTV-silicone the outside portion really good so it doesn't rust.