I bought the "Power Magic Pro" device from the same guy I got the Blackvue DR500GW-HD cameras from. It can easily handle several cameras. (www.blackboxmycar.com in Vancouver)

The cameras and power controller come with much longer wires than you need on average (unless you have a large truck), but rather than cutting and splicing I simply chose to bundle the extra wires together and tuck them up under the dash (protected against anything that might chafe the insulation). In my case I had to splice in a second accessory power socket because I have front and rear cameras. Normally I'd just chop wires and splice everything together with crimp connectors, but I wanted to be able to take things apart in case something went wrong. Because everything works well, I may yet do that and eliminate a big bundle of plugs and surplus wiring.

p.s. ... I like those cameras because I can connect my phone/tablet/laptop with WiFi (password protected) to view the images and recordings, or change camera settings.