My first inclination would be to move the wiring to an easier location to access, or do like electricians do, which is to leave a piece of conduit with a fish tape in it. That way a bad wire could be easily by-passed instead of repaired.
What type of fabric are you going to use? Cloth, Vinyl, leather? Are you going to put the headliner in like the original and butt the panels up to it? The original panels would have been nailed on right through the cloth, but of course you can't do that with vinyl or leather. JCAR's idea is very good, BTW. Face the wood with metal, drill the panel clip holes in the metal, drill holes in the wood directly behind the clip holes, and install the panels like door panels with clips. I wouldn't try to do the panels very big or very long. Here's a '30 Tudor interior. Although you can't see them, there are seams next to the rear window, and in back of the rear windows. In both cases, there are narrow panels above the rear side windows and above and below the rear window. 1930 Model "A" Tudor Interior