Here is a circuit that uses a regular LED. They give component values for anyone wanting to use it on a 6 or 12 volt system.
http://www.uoguelph.ca/~antoon/circ/2qflash.htm
Here is a nice little plip-plop for two LEDs.
http://www.uoguelph.ca/~antoon/circ/flash2.htm
On the bottom of either of these pages is a "Circuits Page" link that will take you to a number of interesting and useful circuits such as this third brake light. It uses only a ground and one connection to each brake light.
http://www.uoguelph.ca/~antoon/circ/3brake.htm
