I cut the wire leading to the stock backup lights and used it to trigger a relay. Then I tied the stock lights and my add-on lights into the relay.
I also put in a manual switch to control the backup lights. Since the manual switch and the backup switch are both tied into the trigger of the relay, you'll need to use a diode on the leg from the manual switch to the relay if you want to use a lighted switch. Otherwise, you'll blow the fuse on the backup circuit whenever you hit reverse.
Also, you can tie the lights straight into the stock wiring, but your lighting will be severely diminished. That's how I did mine at first and the total output was not all I hoped for. After putting the relay in, now the new lights are much brighter.
JM