I won't admit to having to done the same thing before
The usual culprits in my experience are sheet metal penetrations and sockets. If you don't see any obvious wear spots, you might try turning the lights on and wigging the wiring in different places to see if you can cause it.
I had one years ago where the isolation in the socket itself was degrading and it would occasionally short inside. I found it by wiggling the wiring going into the socket.
