(Hopefully) easy question about relays

[matrixq]

Hello,

I found this site while desperately googling around (but not knowing proper terms to get myself some decent results!).

I want to build what I'm guessing (hoping) is a rudimentary circuit, and I"m hoping someone here will be kind enough to point me in the right direction.

The purpose is to take a switched 6v/12v input (i.e. either 6v or 12v goes to the same circuit) and turn that into a switch that activates two separate circuits (i.e. if 6v is coming through, send 12v to circuit A, if 12v is coming through, send 12v to circuit B).

I've read about single pole double throw relays which i think would work if I had 0v/12v switching voltages, but the dropoff is too low to reset the relay when going from 12v to 6v.

Can anyone point me in the correct direction?   

One thing that may be important -- this is in a car and thus 12v doesn't really mean 12v from what I understand.

Thank you in advance for any help!