Most models of iPad require a USB power source capable of 12W or greater (i.e., 2.4A @5V) to reliably charge - whereas iPhone will generally charge (albeit slowly) from less powerful sources of power.
Unless explicitly marked as being capable of high-power charging, many vehicle USB ports - in particular those associated with in-car infotainment systems - may be limited to 5W (i.e., 1.0A @5V).
While your built-in USB ports may not provide sufficient power, you my be able to plug-in a third-party USB charger to a 12V socket in your car. Apple directly market these third-party car chargers (designed for use with a standard automative 12V socket) compatible with your iPad using an appropriate cable:
https://store.apple.com/uk/xc/product/HN7Z2ZM/A
- mophie Dual USB-A Car Charger
https://store.apple.com/uk/xc/product/HN812ZM/A
- mophie Dual (USB-C/USB-A) Car Charger
https://store.apple.com/uk/xc/product/HN822ZM/A