2012 Scion TC Welcome projection Logo lights

I am trying to wire up welcome projection logo lights that will be mounted to the under sides of both doors. When opened this will shine the Scion Logo onto the ground. The problem that I am having is finding a switched source on that will close the circuit when the door(s) open. I have done some research and found most owners have been dropping their headliner and t-tapping into their dome light and then running the power cables down the a-pillars and through the grommet then going into the door cavity. My question is over the weekend I attempted to t-tap into the door switch on the passenger side of the car which I am assuming is + and then grounding the - to the chassis. This didn't provide power to the logo lights but it did prevent the dome lights from turning off until the t-tap was disconnected. What is the best location to tap into power to activate when door opens?