Possibly. I have a PT Cruiser as a rental for a couple weeks and its got an AUX input. I'm assuming if i connect an auxilary cable to the unit on one end and to an adapt for a mini stereo plug on the other end i can connect it to the phone's headphone jack. I'm not sure about the bluetooth thing though.
Yes absoultley you can connect the phone to any HU that has an Aux Input. I have a Moto RZR V3i which has iTunes built into the phone and do this.
You can buy a bluetooth agapter for newer HU's so your Phone will pause the music, Display the phone number calling etc. but I dont know of any HU that can recieve .MP3 music over wireless. Phone to Phone YES! but Phone to HU? That would be cool.