How do I fix the compatibility issue between my iPhone 4S with iOS 7 and my Pioneer car stereo? There was no problem with iOS 6, but now I get a message saying "this device is not compatible" and so I can't use Netflix for example. How do I fix it?

Contact Pioneer. They would be the ones that might need to issue an update to the device to match the current Bluetooth profile on the phone.

On the Pioneer unpair the current iPhone. On the iPhone unpair the Pioneer. Put the Pioneer into Pairing mode and open Settings/Bluetooth on the iPhone to complete the pairing.

This is a typical response from the manufacturer. Did you try the fix that Lawrence mentioned. When Apple or any other phone manufacturer update phone software, they have the latest Bluetooth installed. It is usually the problem with the radio manufacturer that they devices are using the older Bluetooth protocols. You can try this support document http://support.apple.com/kb/TS3581 and see if anything there helps, but generally it requires the radio manufacturer to update their firmware.