Replacing headliner - want to add new lights

I'm in the process of replacing the headliner in my '91 Sport. While there, I would like to add another light in cargo area. I've got a rear-cargo light that came from a '91 Laredo that has a push button on it.

Here's my question:
The new light has three wires (guessing one positive for the push button, one for doors and headlight switch) but my existing harness only has two. Will I be able to splice into the existing harness to run this light, and have the push button work?

I think you're on the right track. I had to do something similar when the switch in my rear cargo light melted - I put a new switch in but can't pull it down right now to check the wiring. If I can get to that in the next couple days I'll try to snag you a pic.

Thanks, if the weather behaves I plann on re-installing the headliner this weekend.

First, I just have to remove all that nasty old foam....