The wireless hotspot is only discoverable when you are actually on the Personal Hotspot screen on the iPhone. If you are on the home screen, or somewhere else, it is not broadcast so the car can't see it to connect to it.
Basically, you have to navigate to the Personal Hotspot screen on your phone, then click on the car icon and then the wireless icon, and connect to the now visible iPhone wireless network. You might not have to actually do anything other than the first step if you are not in range of any other wireless signal.