It could also be down to the vehicles blue tooth. I owned an 06 plate Discovery 3 until recently and it would not "see" many of the newer phones.
One way around this is to purchase one of the FM transmitters that plugs into the iphone 4 (you can buy them that also charge the iphone). All you do is find an empty FM channel in your car radio and tune the transmitter into that channel and you are good to go. The only downside to this is u dont get the phone function, only ipod.