I did this a while ago. Simply peel the rubber away that is tightly attached to the door lining (so the rubber on the door side of that panel, not the windscreen side) and there should be plenty of space for you to thread your wire through. I had mine go under the steering column, up past the headlight, foglight switch then up there inside behind that panel all the way to near opposite the sunroof.
I just peeled the rubber back, threaded the wiring through and then carefully pushed it all back in so that it didn't bend the headlining back and so that it closed with a perfect seal.
I hope this helps,
Pete