The dome light in the Spark EV is an incandescent bulb #T10 type. I replaced it with a 12-LED panel similar to this:
The festoon fits where the original incandescent bulb sat. But then, I'd need a way to mount the LED panel. So, after some thought, I came up with a way to wedge some foam packing material between the LED panel and the light housing:
The LED panel has double-sided tape on the back; the foam blocks stick to that and then the two foam blocks just wedge into the cavities in the lamp housing on either side of the the festoon mount:
Replace the lens, and done!
Tools and materials: