Rust starts at any bare steel, which is usually a rock chip outside, or a seam of the paint elsewhere. Keep any road salts away from those inner door paint seams. The seams you find there typically have a flexible seam sealer underneath, under the truck it's just paint in the seams. When normal body flexing breaks the paint at any seam, the crack allows moisture and air inside, thus rust begins there first.
In any rust prone locations, I'd treat all paint body seams with something that would be flexible and last. Simple wax isn't going to work, help yes short term, but long term find something better. I like under coating for unseen areas, but a visible area like the door jambs, that's tougher. I'd use Ultra Black RTV everywhere that's easy to apply but not visible much. A clear RTV might be worth the trouble in visible areas, but note that stuff does get hard and crack after it gets old or sees high heat. You might get away with doing that and R&Ring it again every few years. Ultra Black RTV will survive the life of the car if you don't physically remove it.
