I have seen this before,door seems closed but light stays on.
The trigger switch for dome light is in the latch now, not like older cars that had the other style. To find which door is doing it,leave door open and close latch with a screw driver or what ever, takes 2 good clicks to close all the way. Then open again with door handle. That should put the light out. Then adjust the striker post move it out slightly. U will need a torx socket to adjust the post.
78 cobra II..... 347 N/A 11.34@119
89 coupe....Alberta's fastest 4 cylinder Mustang 11.40/122.88