To find the correct fuse:
1. Consult your owner's manual: This is the most reliable source. The manual should have a fuse box diagram showing the location of all fuses and what each one controls.
2. Check the fuse box diagrams: There are usually multiple fuse boxes in an E450 (one under the hood and one inside the vehicle, often under the dash). Each fuse box will have a diagram on the inside of its cover. Look for "cigarette lighter," "power outlet," "accessory power," or a similar designation.
3. Use a test light or multimeter: If you can't find it in the diagrams, carefully use a test light or multimeter to check the fuses. Be sure to have the lighter switched on (or whatever accessory is powered by it) when checking. Note that you'll need to check both fuse boxes.
Without your owner's manual or access to the vehicle, I cannot give you a definitive answer. The amperage of the fuse will likely be between 10-20 amps.
