Just like Kloudie1 (Claude) I too drilled a new hole in the bottom of the pedestal. In fact I think I got the idea from him. You can use a hole saw or a spade bit to drill through the aluminum (at least 3/4" in diamater). Watch out because all the aluminum cuttings falls right into your face (and eyes). Have a vacuum handy. The tail on the supplied Garmin plotter was long enough to go all the way from the binnacle down and through the hole. I used double stick foam tape and stuck a terminal block underneath. All the wires (from the black Garmin cable) are inserted in one side. Next is the fun part. Two white cables were required. One was a pair of 16 gauge to supply the power and ground to the unit. That was specified in the instructions. The second was an 8 conductor, 20 gauge bundle for all the other inputs and outputs. I was able to fish them between the fiberglass skin of the aft berth and emerge somewhere near the hanging locker. Then route inside the locker and drill a hole in the wood bulkhead into the interior of the circuit breaker panel. I only used the NEMA 0183 connections. So I had AIS out of my B&G to the Garmin. GPS out from the Garmin went to the B&G and split back to my Raymarine Autopilot. I've attached my crude schematic ( .pdf file below) with the wire colors (naturally the cable from West Marine didn't use the same colors as the Garmin at one end and the B&G at the other). I used another smaller terminal strip behind the circuit breaker panel just to keep things organized. Note that I also split the outgoing AIS signal from the radio and have that go to a DB-9 serial plug then a USB/serial adapter cable. I used to use a laptop and run Open CPN software so I could see charts and AIS at the nav station. Later I got a Raspberry PI computer and with the same software I output it to my 15" LED 12V TV. It's like a giant chart plotter down below. That's been pretty cool too.
For the 9 feet or so separation between the bottom of the pedestal to the circuit breaker panel I needed 25 feet of the cables. You know the saying "you can't get there from here" is never truer. Have fun snaking wire.
Regarding mounting the Garmin 547, I used a teak binocular and cup holder that I bought at West Marine. An additional piece of teak is screwed to the bottom and a half circle is cut to follow the contour of the compass for support. Hope you can see that in the picture. The GPS mount is screwed to that. It works well as I disconnect the cable at the back (I don't use a fishfinder transducer) and open the binnacle clamps to remove the entire thing when we go home. I actually take the GPS off its mount and bring it home. I bought an extra cable on Amazon so I can power it up with the right connector and review tracks or play with the SD card either preloading routes, waypoints and such using the Garmin software on my computer. Very nice for voyage preparation. Although I will say, I liked the old Garmin interfaces better with the Page and Quit key. I could upload detailed tracks via USB to my laptops much easier. The new menu driven interface has not been intuitive to me and I don't get the file storage formats. Maybe it's just me.
I hope this helps.