I recently went on a trip with several friends and we rented a new Ford Expedition. The integrated audio system in the car supported multiple Bluetooth devices with a history (including previous renters) but as far as I could tell not more than once device at a time. Every time we started the car we had to select Sources -> Bluetooth -> Connect -> ${device name} -> Sources -> Bluetooth -> ${device name}. It was a huge hassle and a 3.5mm cable would have been far easier and sounded better.