If they are above 35watts each you need to think about running a power wire directly from the battery. Use a relay that is activated by the reverse light wire that is already there. If you don't you run the risk of burning out that wire "somewhere" and possible taking others with it, or even a fire.
I did the same thing. Running power wires in a Jeep was so simple. There are more than three large rubber plugs in the firewall. Just drill a small whole in one and shove your power wire in it, and then run it to the back along the driver side with the wire bundle that is already there.