The headliner is hard...
it doesn't unzip. It's part of the molded structural liner inside the hull on that vintage of Hunter. Depending on where you need to run the cable it isn't too hard to either fish or push it through between the hull and the liner. If you're adding a radar on the mast the hardest part will be getting the cable down the mast and through the hole in the mast step. It would be a lot easier if you ever unstep the mast. It's worth looking around a bit first, on my 340 there were a few messanger lines left in place that made it pretty easy to pull cables though some spots.
