I use the 12V version from Optronics, it's great. STL78, it runs about 3 watts (running the brake light circuit instead of the taillight circuit) and also is somewhat diffuse due to the number of diodes and their mounting pattern. About $20 as I remember at your local auto parts store (also available online)
http://www.optronicsinc.com/oemtruck3-new.html
http://www.amazon.com/Redneck-Traile.../dp/B001QKVGBU
crappy cellphone pic below, can't tell you the exposure or details, but that's what it looks like in the dark, I don't have to turn and look to see if it's on in the dark, I can see the road reflection of the red beam in my rearview mirror, it's bright.
