Basically, you need firstly to sketch out a wiring diagram for all the wiring needed for this mod, and then go ahead and prepare all the wiring so that you will save a lot of time when the day you install the strips.
A diagram I sketched out that shows all the connections and how everything will be laid out show in blow image.
As you can see in the diagram, the led strip on the passenger side connect to the extra wire length and then get tapped into the led strip on the driver side, then the driver side LED strip gets connected to the extra wire length that goes through the firewall and inside the car under the steering wheel above the pedals. Then a ground wire is tapped-in into that same extra wire length on the - side and gets grounded to a bolt. Next the – and + ends of that same wire goes through the firewall, in the car and goes to the switch, and finally the fuse box tap-in goes from the switch to the fuse box underneath the pedals.
source - instructables