I've seen 50W halogens somewhere that are plug-in replacements for common back-up light specs. The primary caution I would have is that these run hot, and may be too much for the taillight assembly (they might burn a hole in the lens!).
Shop around - there may also be a 35W replacement which doesn't carry as much baggage.
Most of the local auto parts stores carry 50 watt halogen replacement bulbs for the back up lamps. They claim they are "2 1/2 times brighter".
I don't know what type of bulb the back up lamp is, none of the manuals refer to the type, they just call it "18 W". You may want to pull the bulb out and bring it into the store with you.