Hdmi to A/V (pimp my ride style)

[Donnie_Bravo]

Anybody have any suggestions for running a hdmi in (retropie) to an a/v out (head unit of the stereo) in a vehicle? I've seen a lot of people say it's better to do a composite to a/v for most a/v conversions, and I'm willing to try that if it comes to it, but I've already gotten the cables for hdmi to a/v so I'd like to try that first, also the way I am trying to set it up (for cleanliness sake) is an extender with a USB and an hdmi that comes out flush in the center console. (Which I've already installed) Also I'm wondering if there are settings on the pie I can adjust for coding/decoding the signal as I only have a cable that changes the in/out puts and isn't actually a converter, the reason i tried this route first is because I'm limited on my power options and most converters i looked at require an external 5v power source, and I only have one USB that I want to reserve for powering the pie (it seems like it's only just enough juice to run it as the same said USB won't even charge a gen 3 ipod nano I tested on it) I'm hoping for any suggestions from anyone who knows about this type of work better than I do (i.e. basically anybody on here as I'm still fairly new to pie computing) thanks in advance for any suggestions/help anyone can provide and also sorry about the post length (I tried to keep it shorter but i wanted to make sure i explained everything as best i could).