What AB said and here's my 2 cents...
All of the weatherstripping is probably bad, that's easy and fairly cheap to replace. The door striker bushings are probably also shot, they are cheap too. Replacing those two things will make a huge difference in rattles.
Bushings; radius arm and sway bar in particular get worn out. Radius arm bushings are a job if they haven't been before. Sway bar bushings are easy.
Leaks can be an issue, debris in the cowl can cause rust or just block the drain. The drip rails above the doors can have the old seam sealer crack and let water in.
Headlight switches and the plug to the harness sometimes get crusty and/or overheated.
If high miles the driveshaft might be worn out.
IMHO start out by going through battery cables, get those sorted out properly. Then the ground cables. There should be a ground to the frame, engine and body. The frame and engine ground are in the same area (sometimes the same cable) and the factory body ground went from the drivers side cylinder head to the firewall. If the grounds aren't good, electrical gremlins will drive you crazy.