I'm not sure about the S pacl but the Berlina/Calais already has front footwell lights, which is just a T10 globe fitted near the door side in each footwell. Depends on what kind of lighting you want to do.
I have LED strips running in my lancer which i wired up by running a wire from the battery, though you can just use a 12v wire already in the car such as the cigarette lighter's power wire. Mount your strips, ground them, power them. Run parallel wires to the other footwells and repeat. I had them on a 3 way switch so I could control them like my cabin light. Door-Off-On. Have to wire the grounds to your door switches to do this though.
However, in the Holden, I just replaced the T10 bulb to a blue high output one and will probably run wire from the front lights to the rear and just use SMD strips. Will cost about $10-$20 for the lights.
If you want to get it professionally done. Talk to Phreddy from ASR. You can find him in the sponsors section.