Where is the backup light switch on a 2003 E350 van?

There isn't a separate backup light switch on a 2003 Mercedes-Benz E350 van (or most modern vehicles). The backup lights are controlled by the reverse light switch, which is integrated into the transmission. When you shift into reverse, the switch activates the lights. Therefore, there's no switch to adjust or replace independently. If your backup lights aren't working, the problem likely lies within the switch itself, the wiring, a blown fuse, or the lights themselves.