I have just bought two rear brake/side light LED bulbs from Classic LED's.
They are a direct swap for the old bulbs and draw far less power, so place less strain on the wiring.
You have to buy red ones, not clear, for the full effect, and they are brilliant in all senses of the word. At 7 quid each they are well worth it, as my old brake lights were pretty dismal on a sunny day.
Great for helping to stop the old lady in a Fiat 500 running into your back end