Door open

My door open light loves to go on when I hit a bump, sometimes it stays on until the next bump. Sometimes it doesn't come on for weeks. I know the obvious thing to do is to change the door switch but I don't buy changing parts until you find the problem. How do I troubleshoot this problem. This is on my 1995 525i Touring.

Dismantle the central light until you can see its circuitry. It should be fairly straightforward once you look at it...first you will have to remove the translucent cover etc. Some of the contacts may need to be cleaned, or prised back into position. Might have loosened out over time.

The door switch needs adjusting/cleaning/replacing.