I spent some time last season running a lot of wires on my Ď03 2900. Probably similar config. On my boat any routing across the beam is tricky. Iíd recommend the headliner. 1) remove the shelf in the head to get access to the bundles going to the dash. 2) remove the cover over the wiper motor in the dinette. 3) pull the ac panel.
The best route is likely from the dash, around the head (over the top or around the gunwale), then over the headliner to the panel.
Not 100% sure but think the actual route for the radio and ac cables across the beam is down under the cabin steps, under the floor behind the fuel tank bulkhead and up to the helm from under the head. This path appeared impossible to do a retrofit, which is why I recommended the headliner.
Best of luck.
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