Welcome to Android Central! First, when you connect to the PC via USB, swipe down the notification panel and look for a USB icon there. If you see one, tap it and make sure MTP is selected. You should then be able to access the phone's storage directories using Windows Explorer.
Second, keep in mind that unless your car has integrated Android Auto, connecting the phone via the USB port often won't do anything. The USB port in a car has traditionally been used for music stored on USB flash drives. Some phones (mostly Samsung) can output audio via USB, but the phone won't be treated as a USB drive. Regardless, you wouldn't be able to connect it via USB as a GPS device unless you have Android Auto.