Rear work lamp
Actually two for sale, either is perfect for mounting on the rear of your roof rack but obviously could be mounted wherever you wanted more light. I never got around to hooking either of these up but just tested both and they work and are nice and bright.
First one is a CAT heavy duty lamp by Nordic, cat as in caterpillar, has an excellent vibration resistant mount clamp, once you put this where you want it and tighten it down, it will not shake loose. Housing is sealed watertight, has a 55W halogen bulb, this is a very cool little light. Perfect for a TDI truck... $30 shipped
Second one is a plain aftermarket style lamp, was on my rack when I bought it, never been hooked up I dont think... $15 shipped