Originally Posted by willwren
Consider the possiblilty that you'll need a larger fuse, and that the gauge of wire won't be sufficient to support the current demand of all four lamps.
I was thinking the same thing. To be specific, do _not_ try to power one set of headlamps (e.g. lighting up the low beams...) by just piggybacking a connection from the other set (...when the high beams are on); you will double the operating amperage on whichever circuit is providing the power at that time.
If you want to add a relay (such as the J.C. Whitney kit), that'* fine, but the circuit brought in by the relay to light up the second set of headlamps should be a separate fused power supply, not just a jumper lead from whichever headlamps are currently on.
(Yes, that can be a fused power supply for the relay, it doesn't have to be a circuit breaker, since if the fuse blows, you'll still have the car'* original circuit breaker protecting the original power circuits to the headlamps.)