That set up, while possible, requires a very important thing: That your device supports HDMI output through the USB-C port. Not all devices do. (Plus that set up kinda defeats the purpose of no-dongle)

Chromecast, unfortunately, doesn't support direct connection so you'd need a router in your car to hook them up (or use your phone as a router and cast that way). I believe Rokus are the same. Problem with a Chromecast and most Rokus, is that they are all HDMI. Only the most basic Roku supports composite output (not RCA, IIRC), so you still need the HDMI converter most likely). You can search online for casting dongles that do support WiFi Direct connections (so you don't need a router), but I haven't seen any that support RCA, to be honest.

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