I've tried a few eBay festoon LED lamps for the car interior. Problem is almost all of them are not very good.
They are either dim, don't fit or run hot or any mix of these issues. My limit was the small 31mm lamp does not allow much to compete with the stock 5W bulb, i.e. 6 LED's is not enough, and a Cree XML and boost converter still needs to get rid of a lot heat in an enclosed fixture.
What worked best for me is SMD 5050 LED arrays with the festoon adapter. Even then, some run quite hot and I had to put in bigger resistors. I stuffed in the biggest board that will fit. I also silicone'd one in place and just soldered wires directly.
Biggest I used is 48-LED white SMD5050, each LED is 3 chips around 9V. Originally 16 of 120R 0805's to give 30-45mA per LED at 12/14V for over 86C too hot for my liking so I went to 200R to get 20-28mA which runs cool. The pc boards are dual 6x4 arrays so you can snap it in two for smaller fixture.