I'm lucky in that the previous owner kept or put in a teak lip around the "shelf". Just going off memory (because the boat's at the shop), I poked through the headliner from above the deck w/ a philips screwdriver. (Put a small piece of plywood on the cabin liner so it stops the screwdriver.) Here's my cuddy before I rewired.
I ran the wire down to a plastic wiring clamp (screwing that in was fun with this stubby little screwdriver) on that shelf and ran the wires flat on the shelf to another clamp just behind that right, inside "corner" of the trim. The wires then just run behind the wood lip and then up behind the dash. How it's routed there I forget.