When I found a LED circuit board that had an input voltage of 4v-18V, I thought it would work well as a driver for an automotive LED bulb. The circuit board uses a Micro Bridge Technology PT4105 (PDF specs) step down LED driver.
I took an 1156 bulb to use as a base for the circuit board and LED. I broke out the glass and filament and used JB Weld to glue the components together.
The LED was brighter than the 1156 bulb, but it had a noticeable hot spot, even though the Cree LED has a 90 degree viewing angle. Heat doesn’t seem to be a problem; the Bavaria’s light socket dissipates the heat well.
Joe Weir, on the Senior Six Mailing List, suggested using a diffuser lens. I’ll have to find something then report back.