Use a dual postion swich with accessory power running to one pole of the switch and dome light power running to the other pole of the switch. Then the common pole of the switch is wired to the coil on the relay. The lights would receive power directly from the battery only with the relay closed. The relay would be opened or closed by the switch in the accessory power position (closed), off position (open), or dome power position (closed). I have a very similar setup on the reverse lights I just installed. You would just need to splice into a wire that powers an accessory and a wire that powers a dome light. I'm working on a wiring diagram for my setup that should help you. It's nice to have the wiring diagrams too so you can trace back to convenient splice points.