I don't think the electric system of the Tdub can handle any extra incandescent bulbs. Maybe if you can find a LED solution....
On a recent trip my buddy had an electric problem with his head light (the switch malfunctioned) so we put a couple of LED flashlight tight with ziptights in his side guards. We were extremely surprised that you could see them even in daylight and they provided enough light to get us to a local shop to fix the switch.
Some of the new CREE LEDs are very impressive.
Maybe you could use a halogen fog light housing with these LED Lamps. They use only 2.1 watts.