The switch is built into the light housing. I came up with a way to add lighting using the factory switch, was going to do a write up on it but never got around to it. Let me see if i can find the pictures and post about it tonight.
Here is what i did, short but to the point. I don't have any pictures of each step but it should be pretty easy to visualize. The factory trunk light housing has a built in gravity switch. I took the existing bulb, covered it with a rag and broke the glass. After cleaning out the inside of the bulb i drilled a hole through the center contact point big enough for the stripped end of a wire to go through. Strip about 1/2" of insulation off the wire and slide in through where the bulb used to be and through the hole you just drilled. Solder the wire from the outside then snip off the extra wire. Try to make the soldered area close to the same size as the original contact point the bulb used to have. I mixed up some 2 part plastic putty and while holding the wire in the center filled the area where the bulb used to be and built up enough outside the metal part so you have something to hold while putting the bulb back in. The wire sticking out will be the positive to hook up to any extra lighting you want to install. Remember not to overload the existing circuit.