capacitor ?

Hey guys looking for some help on my e90. I bought a HID kit from DDM with code error and on the install only 1 headlight would stay on and they would flicker and still get error code on dash. So after some research I found out that everyone one uses capacitor but the guy at ddm told me I have to Tao in to my car Harnest is this true ? Or can I just tap in to the bulb Harnest kit instead since I don't wanna splice in to the car it self thanks guys