No current auto GPS offers both bluetooth and a cable connection for a bike's intercom system. As you already noted, auto units are not waterproof. But the single biggest detriment is that no auto GPS is daylight readable so they cannot be seen well at all during daylight hours under any circumstances, and not seen at all in direct sunlight.
Maybe discussions on this topic if you do a search. What I usually recommend to people is to use the automotive GPS if you are a casual user and do not count on the unit actually working- like it is more of a trinket than a tool. Those who need a GPS that works and is usable on a motorcycle really need a motorcycle unit unfortunately.
Brian
I don't want to sound like an unwashed heathen, but does anyone just take their automotive GPS and Velcro it to a bracket? I know it would not work in the rain, but otherwise for us cheapskates, is it an option people use?