I just did this same upgrade in September of last year.
First off, I couldn't find any maker that that used the same 3 wire hook up and allowed the audio directions to be fed into your intercom. It can be done, but takes a bit of "******* Engineering".
We had already switched to a Scala Bluetooth intercom so that did make our choices easier.
The previous owner of my motorcycle had installed a BMR shelf and had moved the NAV I (SP III) unit there and the wiring was coming out of the map light in the dash, but did not use the map light wiring. The GPS had it's own wiring.
I had decided on the Garmin Zumo 220 for a number of reasons. It's motorcycle rated and friendly from Garmin, it's waterproof (no zip lock bag needed), comes with the Mapsource DVD, comes with the wiring harness for a motorcycle and comes with a very good motorcycle mounting unit (you MAY need one more part from RAM). It's basic, it doesn't do phone or music but does use Bluetooth to send directions to your BT intercom. At the time the complete unit from Garmin was $400.00 for the complete package for everything you need.
Here's what I did. I checked the voltage on the existing GPS wiring and had 12v DC. The wiring kit for the Zumo has instructions for going to the battery so they're figuring 12v. I merely snipped the existing wiring and connected the new 2 wire hookup supplied with the GPS. I now had switched power for the unit. The 3rd wire I just snipped and capped. All my wiring is right back where it was, in the dash. I do not regret my decision one bit.
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