I'm using LED lighting in my Insight. I'm now on the third revision and working on a fourth revision.
Currently I have 30 LEDs soldered on to a PCB with an active current limiter composed of discrete parts.
The cool white light from the LEDs is noticably brighter and less yellow than the OEM incandescent bulb. The prismatic lens on the dome fixture does an effective job of difusing the light in the cabin.
The problem with LED lighting is that the LED works over a very narrow range of voltage. Typically a white LED is off at 3 volts and burns out at 3.5 volts. The voltage to the dome light varies quite a bit. When you have the engine off and the door open the voltage can be several volts lower. Not taking this in to account I burned out my first set.
To prevent this the voltage or the current must be limited electronically. A typical three terminal regulator has a voltage drop of 2.7 volts. The voltage across the bulb socket could be as low as 10 volts, so we are left with roughly 7.3 volts. This gives enough head room for two LEDs in series. Naturally we can add an unlimited number of series sets in parrallel to create more light. To achieve the correct voltage the three terminal regulator must be an adjustable type or we have to use one or more diodes in the sense leg to achieve the correct voltage for the LEDs. The problem with this approach is that as much as 50 percent of the energy is wasted by the regulator.
Currently I'm working on a switch mode regulator that should be 80 percent efficient. I'm planning to increase the number of LEDs as well. This is a fun project and I'm hoping to get dazzling results.