How to connect an iPad to a car stereo system
USB input and Bluetooth are the two most common ways to connect an iPad to an OEM or aftermarket car stereo head unit. I personally chose the Sony RSX-GS9 because it has a rear USB input that supplies enough wattage to keep my 12.9" iPad Pro charged and has the flexibility to support further upgrades.
The RSX-GS9 lives behind my iPad dash panel and is operated by my OEM steering wheel controls by way of a PAC Steering wheel control adaptor. Steering wheel control interface lookup
If your car stereo's USB input doesn't supply enough wattage to charge your iPad (or if your head unit behaves undesirably when put into "iPod mode"), you can use an Apple Lightning to USB 3 Camera Adapter, Apple USB-C to Lighting Cable and Belkin USB-C 36 watt Car Charger to enable fast charging and a USB Audio output that is compatible with most aftermarket and OEM head units that support USB Audio.
*A side note for audiophiles! The Sony RSX-GS9 supports High-Resolution Music file playback over its direct USB DAC connection with a maximum sampling rate 5.6Mhz. I recommend using the Onkyo HF Player iOS app for native DSD Music file playback over the RSX-GS9's USB DAC input for highest audio quality.
Other recommended head units
Sony MEX-GS820BT - Front and Rear USB with Bluetooth
Pioneer DEH-80PRS - Dual rear USB with Bluetooth