Dr J is on the right track here. This is exactly how I make my tail lights for running/brake. I run LED's, so you're running resistors anyway if you make the lights yourself. The lights you buy from the store have resistor networks inside already on the little circuit boards, this makes your life easy. If you have a decent multimeter and know how to use it, you're golden. Most decent sized LED's should draw roughly 20-30mA each, depending on how everything was designed. If you add resistance (resistors from Radio Shack or wherever) in the running circuit, this will bring your voltage down, thus reducing the current through the LEDs according to Ohm's law (look it up, it's pretty easy to understand). When you apply the brake, you give the light full voltage and thus, full current making it brighter. I've found that if you reduce the current to about 2/3 of what it draws originally it will work out to about the right difference in brightness. So, if the light out of the box hooked up to a battery draws 60mA overall, then the magic number you're shooting for is 40mA. You can adjust this by one of two ways. 1. Keep buying different sized resistors until you get it right, or 2. get a bunch of the same size and add them in series (end to end) to reduce current, and in parallel to increase it. I've found that around 400ohms is a great number because this gives you 30mA at 12V. Once you've found your magic combination, solder it all together and wrap it in tape or use some heat shrink.
