Apkano is right. LED's as a retrofit can be a bit tricky. The direction the little lamps put out their light and the way it reflects from the inside reflective surfaces is hard to guess.

www.superlumination.com Tail Light Brake Light Turn Signal LED Bulbs

www.autolumination.com Tail Light Brake Light Turn Signal LED Bulbs

These guys sell a bunch of different styles of side-firing and end-firing types. And combination of the two. I think we need side-firing but it can get pretty expensive to guess wrong! Apkano's guy probably had the wrong style.

And don't forget the large load resistors you will need!