Check the wires coming off the plug that goes to the lamp board going back in the cabinet.
My friend has a bunch of lights not working. And the stop light was out as well.
Turns out the wires were crushed under the playfield near the back.
Check both sides.
The wire for that and other lamps is real close to the side slide rails and can get crushed and broken from moving the playfield for service.
If all else looks good. I bet that the problem.