how to trace an electrical lighting problem?

My '93 Safari has a quirky interior lighting problem. The interior lights sometimes go on and stay on while i'm driving or parked. Luckily I can turn them off with the master switch on the dashboard(above the radio). I had my mechanic look at the problem and he couldnt find anything wrong. I replaced two of the door switches and the dash master switch in an effort to try and repair it myself.....but to no avail. For awhile it was ok and the lights wouldnt stay on, but then all of a sudden they stayed on overnight and killed the battery. Now I keep them off all the time so as not to risk getting a dead battery again. What can i do to try and trace this problem? It almost seems to me like its short somewhere in the van........but how can I trace it?

is it all of the interior lights or just one that stays on?

its all of the lights that stay on

replace the door switches first, probably a problem with one of them. cheap and easy