21v ground?? where?

alright, I am going to wire up some aux lighting in my truck (its actually one of those fake alarm lights) and i want it to come on when i shut the engine off. The instructions say to wire the red and black to positive and ground (duh, right) and then it says to wire the white to the 12v ground that comes on when the vehicle is turned off...just wondering where/what exactly it is? thanks in advance!

I did that once and used a positive that shut off when the key went off because a power will go ground when not active,I think I used the fuel pump feed,but if that was before this morning,the memory might not be so accurate.

not really sure what your saying..anyway i have it wired up except the white...i have no idea where it goes!