you need to use a switched signal, liket the one that used in the dome light.
I'm not sure of where the wiring for the dome light goes through, but once you find it, you can tap into it and then run a relay and send power to the door sills. very easy mod, finding the dome wires, >b****.
the door sense wire is inside the door, it's not a pinswitch on the frame. You'll have to dig around in the kick panel to locate it coming out of the door boot into the cabin. If you can find an alarm wiring diagram for the 3, it will show you the correct "door trigger" wire to tap into.
And ya, a relay is a good idea and be sure to use a fuse and use the correct amperage. If your not sure, start with a 5amp.