Biglew,
The plug in to the Radio aux is for the sound through your stereo, as in MP3's loaded on the zumo. Garmin's allow for operation on battery or via 12v DC. If you get the auxillary power (cigar lighter) from harley (about $19.00) it comes with a plug that plugs into the "accessory port" located directly in front of your battery; you can then attach the cigar lighter and of course your 12v adapter. Whatever you plug into will be controlled with your accessory switch located next to your cruise switch.
I use a Gramin 765T and the 12v adapter is also the antena for live traffic updates (comes with the 765t for life for free). The 765T is not waterproof however i rarely ride in the rain and when I do I can put it away or cover with a ziplock bag.
Buying.... unless you have "harley bucks" to spend shop it around on the internet, just make sure you buy from a Garmin authorized dealer just in case you need the warranty.
I would recommend that you shop GPS units and find the one that you will live with as there are many features, whistles, and bells - don't buy something you'll never use. I personally prefer the Garmin units (although not the Zumo). I attached the comparison from Garmin's site on the 765t and the zumo 660.
I hope this helps.
Mac...
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nüvi® 765T
$ 0.00 USD - $ 449.99 USD
zūmo® 660
$ 0.00 USD - $ 799.99 USD
Unit dimensions, WxHxD: 4.8"W x 3.0"H x .8"D (12.2 x 7.6 x 2.0 cm) 5.3"W x 3.3"H x .9"D (13.5 x 8.4 x 2.3 cm)
Display size, WxH: 3.81"W x 2.25"H (9.7 x 5.7 cm); 4.3" diag (10.9 cm) 3.81"W x 2.25"H (9.7 x 5.7 cm); 4.3" diag (10.9 cm)
Display resolution, WxH: 480 x 272 pixels 480 x 272 pixels
Display type: WQVGA color TFT with white backlight WQVGA color TFT with white backlight
Weight: 6.48 ounces (183.8 g) 9.5 ounces (270 g)
Battery: rechargeable lithium-ion removable, rechargeable lithium-ion
Battery life: up to 3 hours up to 5 hours
Waterproof: no yes (IPX7)
High-sensitivity receiver: yes yes
RoHS version available: yes yes
Preloaded maps: yes yes
Ability to add maps: yes yes
Built-in memory: internal solid state internal solid state
Waypoints/favorites/locations: 1000 1000
Routes: 10 20 (additional storage on microSD™ card)
Basemap: yes yes
Accepts data cards: SD™ card (not included) microSD™ card (not included)
Voice prompts (e.g. "Turn right in 500 ft."): yes (internal speaker) yes (internal speaker)
Speaks street names (e.g. "Turn right ON ELM STREET in 500 ft."): yes yes
Voice-activated navigation (operate device with spoken commands): no no
Lane assist (guides you to the proper lane for navigation): yes (with junction view) yes (with junction view)
3-D building view (displays buildings in 3-D): yes yes
Auto sort multiple destinations (provides most direct route): yes yes
Auto re-route (fast off-route and detour recalculation): yes yes
Choice of route setup (faster time, shorter distance, off road): yes yes
Route avoidance (avoid highways, tolls etc.): yes yes
Bluetooth® wireless technology (connect to your phone for hands-free calling): yes (with A2DP technology) yes (with A2DP technology)
MSN® Direct compatible: yes yes
FM traffic compatible: yes (receiver and lifetime traffic included) yes
XM® Navtraffic & Radio for U.S. compatible: no no
Speed limit indicator (displays speed limit for most major roads): yes yes
Where Am I? (find closest hospitals, police & gas stations, nearest address & intersection): yes yes
Garmin Locate™ (marks position when removed from windshield mount): yes no
ecoRoute™ (calculates a more fuel-efficient route) : yes no
Qwerty or ABC keyboard (choose keyboard layout): yes yes
Multimode (switches modes to drive, hike, bike or boat): no no
Custom POIs (ability to add additional points of interest): yes yes
Garmin Garage™ vehicles compatible (download car-shaped icons to your device): yes yes
Garmin Garage™ voices compatible (download custom voices to your device): yes yes
Photo navigation (navigate to geotagged photos): yes yes
World travel clock, currency & unit converter, calculator: yes yes
Picture viewer: yes yes
MP3 player: yes yes
Audio book player: yes yes
FM transmitter: yes no
Headphone jack/audio line-out: yes yes
Remote control: no no
Garmin Lock™ (anti-theft feature): yes yes
Touchscreen: yes yes
Dead reckoning: no no
Motorcycle-friendly: no yes
Trucking-friendly: no no
Marine-friendly: no no
Geocaching-friendly: no no
Additional: This USB mass storage device is compatible with Windows® 2000 or later and Mac® OS X 10.4 or later. This USB mass storage device is compatible with Windows® 2000 or later and Mac® OS X 10.4 or later.