So you're basically trying to turn the N810 into a glorified bluetooth receiver for your car. I can picture what you're trying to do, but even if it were possible, it's soooo much hassle.

It would really be a lot easier to just have a bluetooth ear piece that stays in the car in your ashtray (or in one of the 10000 cupholders all new cars seem to come with) at all times and you just put it on your ear when you receive a call.