This question extends from lack of understanding of exactly how a relay works.

I want to buy some cheap lights to mount underneath my rear bumper for when I'm backing up (cuz I'm cool like that), something like this (http://www.harborfreight.com/cpi/cta...emnumber=37537).

Is there a way to make the reverse lights act as the switch to turn these on?

Kind of like this, just vampire into the pre-existing wiring?

Also can you buy these relays individually? I haven't really looked, just curious.

 

Connect the "green" wire in that diagram to the reverse-lamp circuit.

 

^^ x2

 

And they're DIN relays, commonly known as "Hella" relays behind the counter. You should be able to find them pretty much anywhere.

 

Sweet, thanks guys! So it doesn't matter which side of the circuit he green wire is connected to?

 

The green wire in the diagram needs to be connected to the power-side of the reverse light circuit.

 

No. The coil inside will work with reverse polarity also.

 

Well I put it on the + side and it worked fined, actually worked the first time too, no bad grounds or anything!