Sailor,
depending on what the headunit is, there may be an auxillary cable (or box-type reciever) to directly connect to headunit in the car, otherwise you’ll need to use the FM modulator to broadcast to it. Broadcasting kills some of the soud quality, but then again what we listen to is marginal sound quality anyway.
For the garage, you could pick up a Ipod boom box for under 50 bucks, which has it’s own dock and built in charging. I see them all the time at BigLots and stores like that. I hve one in my garage as well