I need some help creating a relay circuit for my revese cam.

I am using a device called carbuttons - this converts resistance into keystrokes.

I need to send a momentary pulse when the +12 is detected from the reverse light, and then send another (differant) resistance pulse when the 12v is removed.

i have seen this diagram to send a pulse, but i cannot work out how to combine 2 and discharge the caps immediatly.

http://www.the12volt.com/relays/page5.asp (1st diagram).

Any help would me much appreciated!