OK, I finally got the wiring installed (or at least the cable run and ready to connect). All in all, I think poking myself in the eye with a sharp stick would have been more fun, but it’s now a nice looking and clean installation. Since it took quite a bit more than expected, I thought I would post the instructions so anyone else crazy enough to try this will have a somewhat easier time.
This is for a 2018 Spyder RTL with standard (not Triaxial) handlebars. The GPS is a Garmin Zumo 595, although the 590 may be the same.
1: Once the tupperware is off, pop out the rectangular plug in the center of the steering column. A small screwdriver inserted at the top (forward) point of the plug will pop it out. Do not pry from the side or bottom or you may damage the plug.
2: Pop off the cap on the inline fuse in the GPS cable . This will make is somewhat smaller and marginally easier to thread through the hole.
3: Push the red and black (power) wires through the pop-out hole on the steering column. There’s a lower hole inside the column towards the aft part of the column, and the wire should go through there. Looking from the right side of the bike you should see the wire, which is (of course) just out of reach. You’ll need some sort of ‘grabber’ to snatch the wire and pull it through.
4: Remove the rubber cap from the USB cable, which will make is somewhat easier to thread through. Insert it in the hole and through. Once the end is through the first hole you may/will need to rotate the cable 90 degrees since the pop-out hole is rectangular front to back, while the lower hole is rectangular left to right (how nice!). Pull the USB cable all the way through.
5: Pull the red and black power cable almost all the way back out the pop-out hole. Make sure not to pull it all the way; leave a short amount sticking out the right side of the bike. This will give you about 8” of red and black wire sticking out of the pop-out hole.
6: Take one of the audio plugs and tape it to the power wires, right next to the pop-out hole. Push the taped audio/power cables through the pop-out hole while at the same time gently pulling the red/black cable outwards from the right side of the bike. This should result in the audio cable coming out the right side with the power cables (unless the tape rips, in which case you go back to step 5 and start over.
7: Repeat steps 5 and 6 for the remaining two audio cables.
8: Last but not least is the big rubber contraption that merges all 6 leads into one cable. This has to be inserted into the pop-out hole and pulled through also. Unfortunately, the pop-out size is 1.14”, while the rubber thing is 1.16”, so it won’t fit. My solution was to take a Dremel tool and grind down the rubber. Be careful not to hit the wires when you do this, of course.
9: Once you have mangled the rubber thing so that it fits through the pop-out, you’ll find that it’s a bit too long to allow you to rotate it and push it though the bottom hole (remember, you have to rotate it 90 degrees to get it through the lower hole).
10: Loosen the handlebar bolts. Top bolts just a little, and bottom bolts a fair amount. No need to take the handlebars off, but you do want to be able to lift the bars a bit. The bolts are VERY tight; probably held in with red Lock-Tite, but they will (eventually) turn.
11: Rotate the rubber thing 90 degrees and push it and the remaining cable all the way through, re-tighten the handlebar bolts, and the cable is now ready to be connected.
12: The pop-out cover no longer fits, so you can either leave it off or cut out a U-shaped section to fit over the cable. I haven’t done that yet, but how hard could it be (and what could go wrong? :gaah

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Still have to route and dress the cable, as well as putting a connector on the power leads and hooking it up to bike power. That’s for another day though; it’s now time for a beer.
Hope this helps someone who is considering installing a Zumo 590/595 on a 2018 RTL and wants it to look good. Many (many!) thanks are due to h0gr1der for all the suggestions on this task. Without his help I would have just routed the cable though the space at the bottom of the steering column, which is not really a good solution since it looks bad and may be subject to chafing.
- Ken