Hi Bruce, no worries, we all started from the beginning!
Correct me if I'm wrong but I believe the Garmin 220 has a so-called active GPS-holder.
Meaning, that there are electronics inline (read: in the cable, inside the cigarette-connector or inside the cradle) and that also could "trigger" the can-bus.
But there could also be a rather easy way to check if the battery of the Garmin 220 is the culprit;
You may perhaps notice the charge-indication on the battery-level indication versus a power-on indication on the battery-level.
Meaning, you will see a charge-in-progress versus an indication of the battery-full indication.
Often Garmin's do show a difference. So that you know that the battery is charging or if it is fully charged. (and will then often show that an external power-source connected).
But as I do not have a Garmin 220 you may want to look in the manual, and observe the battery-level indication icon on the Garmin 220 with a fully charged battery and a partially drained battery.
Then a firm warning, if I understood you correctly;
Please do not hook-up any (!) electronics or loads onto the tail- or brake-light of any BMW.
The ABS and the braking system (and the cruise-control, if present) are hooked into the ZFE (the central computer of the BMW) and putting additional load (or changing it, e.g via replacement of the lightbulbs with led-versions) will cause problems.
This is a proven fact and you should stay away from the tail and brake-light. Please.
On the 1200-series, the brake- and tail-lights are modulated, meaning voltage is switched on-and-off very rapidly to emulate brake- and tail-lights using only one filament.
Putting any load or changes of the bulbs will affect the ABS, braking system and cruise-control.
If you want switched power you will need the aforementioned fuze block (et al) so that whatever you hook-up to the bike, it will not interfere with the electronics.
Unless I misunderstood your attentions, and I apologize.