I have put LED lights in the tail lights of my Firebird. When I did my research before choosing which LED:s I should buy, I found that the cheaper brands (chinese?) had very little difference in brightness between the tail light and the stop light (or turn signal). The tail lights were so bright that it was too little difference when the brake light also lit up.
I found that people who had bought Sylvania ZEVO or Philips Vision LED:s were more satisfied, because they have a more stock looking difference in brightness between the tail light and the stop light. I ended up buying red Sylvania LED:s from eBay. To get the right frequenzy on the turn signals, I soldered some wires to the sockets and soldered the other end to the original bulbs, to get the right resistance. The old bulbs are tucked away on the sides of the trunk, below the side marker.

I'm happy with the results. The tail lights are slightly brighter than before, but most important, the brake lights and turn signals are brighter than before. Hopefully that will keep me from being rear ended.

Maybe I should add that here in Sweden we have to drive with the lights on all the time. That why it's so important to have the difference in brightness between the tail lights and the turn signals, so the person behind me can see my turn signals well.