im trying to put a raspberry pi as a airplay server in my car. i have a ipad as a display in there now hooked up to the existing head unit via 3.5mm aux cord. i'm looking to have stereo input on the raspberry pi that is connected to the ipad and have a script that makes the ipad as the primary output to the speakers and if the airplay server is connected then that'll be the sound output. to allow for the airplay server to work i have an wifi access point that is not connected to the internet forcing any phone that is connected to it to use LTE instead of trying to use the access point as an internet connection. this brings up one problem, when the i have my car parked outside my house my phone connects to it instead of my home network. to fix this i plan to have a relay on the 12v ignition controlled wire and use one gpio pin to make the access point active and inactive based on the state of the car. so now im trying to find a cheaper usb or gpio sound card so add a sound input as well as an output because the 3.5mm output on the raspberry doesn't have great sound quality. my problem is the only usb sound cards that I've found don't have a stereo input but only mono inputs and if they do theyre upwards of 100 dollars. ive found the gpio sound card audio injector zero from kickstarter which is exactly what i need but i dont know if thatll not allow me to hook the relay to the raspberry pi and the network will always be on. does anyone know of a usb sound card that has a stereo input that is decently cheap or if the audio injector zero gpio sound card will allow to use one of the gpio pins for the replay?