Okay, we have multiple challenges on this van. First, has the van been wrecked at one point? If yes, there is a good chance a part of the wire harness was damaged by metal parts crushing the harness. This will explain erratic behavior when the wires move around. Again, if this is the case, the best solution is to replace the harness. Another solution is to identify and replaced all bad wires only. However, this takes more time.
In regards to the tester, what kind of tester do you have? Is it a multi-meter? Please let me know brand and model and perhaps I can find if on the internet to give you some tips.
In the mean time, my suggestion is to start isolating the door switches. For this, remove each door switch and verify their state of corrosion, broken studs, electrical wires, etc.
For your front headlight, visually inspect the connector and look for signs of overheat, high temperature damage, or burnt connector. If this is the case, you will have to replace the connector and few inches of wire.
Let me know what you find.