Before tearing apart the pop bumper, there are a few things you could also try:
1: If you have a spare lamp socket (any socket) you could desolder the lamp socket pins currently in the machine, alligator clip the lamp wires to the spare socket, and see if the lamp lights.
2: If the spare socket lights, you know the wiring is good and be sure that there's something wrong with the pop bumper lamp socket.
- If you get lucky, it's a cold solder joint and re-reflowing the solder on the pop bumper lamp socket makes it work again.
- If reflowing the solder doesn't help, you need to take the pop bumper apart and replace the socket
3: If the spare socket doesn't light, perhaps you have a broken wire or corroded pin in a connector. Confirm where the wire goes with the schematic, check for point to point continuity, and repin the connector header if necessary.