Car wont start unless door is open

Do you mean like using a multimeter?
Yes. Most of these circuits are not that complicated. It's when melted together harnesses "get together" that stuff can happen. You need a combination of test lamp, multimeter, and just plain unwrapping suspect harness sections to inspect. Often though, in the case of "melted shorts" you can feel, flex and visually examine harness to get a clue.