Just bought about 1000 white and 1000 blue leds on e-bay and my first project was in my camper. One of the sockets broke off (thanks son) so I deiced to try out some white LED as a replacement. I used a Vector Board also know as pref board or matrix board or I've even seen Prepunched Insulating Breadboard. There are 30 LED's and it's about twice a bright as the 921 bulb, but very directional, the 921 has more of an even spread.
A branch is three 3.2V 20mA White LED in series with a 120 Ohm Resistor, Nine branches total at about .18 Amps @ 12VDC. A 921 Bulb take about 1.33 Amps @ half the brightness of the LEDS. Pics of the final install in the camper coming soon, we will see how it looks :)
comparison between LED and 921