HiTech has a nice product but very pricey. White LED's are the priceyest to produce for reasons I can't really explain, I just know they are. But $180 for a circuit board with x amount of led's seems to be too much.
The type that replace the bulb for a small cluster of LED's do have some drawbacks with certain taillight configurations as mentioned above.
If you are crafty and have some time and a bit of $ to waste, you can build your own set like hitech from parts from radio shack or on line vendors that sell LED's, computer case modders are a great source, seeing how they run off both 5 volt and 12 volt, 12 volt being better suited for us due to our 12 volt car systems. You can use 5 volt, but will have to put a resistor in line.