best way to power rear camera 07 silverad

i want be able to see cam while driving and the bottom left tail light i hooked into only powers if i manually turn driving lights on.   it wont power up w the light switch on auto even though its hooked to the day light running lights. I guess the voltage jumps when  the switch is on? is there a better light that is on constanly that I should use? or just pull power wire to the 12v behind the radio. also there are what looks like hum bars accross the cam the way it is hooked now. is that normal when powering off of tail lights? please advise. thnx

07 silverado 1500 new body with bose prem system. inexpensive camera with night vision. The tail light i hooked into had three wires going into it 1. yellow. 1. brown . 1. black . it is not wireless. I pulled video to cam and powered locally at tail light. I used my volt meter on the wires but it was hard to get a consistant read. I figure that maybe the voltage is the but amps might jump once the lights are turned on manually. idk im sure I can get it figured out given enough time but I;d like to hear from someone with some exp on that truck, etc.