You might also benefit from some RF shielding on both the Pi and the cable. The Pi throws off a huge amount of RFI , to the point where even a cable plugged right into it is going to catch some too. Use a grounded case on the Pi (or just put it in an Altoids tin with standoffs) and a grounded plug too. Don't forget to ground the Pi case to the car's ground too.

@mukelarvin Well what i use in the video is the cheapest, but as of late I've been using a VGA to Component transcoder, theres a 15khz distro of retropie that someone made that works like a dream. they use some kinda data base for the customs timings, but I do like the RetroTink, its less of a mess when it comes to wires, but its about 90 bucks and needs some time to set up. So the HDMI to Component is kinda the poor mans way but it does work , it has its small problems , overall its a good starting point .. 12 bucks + 5 or so for the cables. for the vga666 i think i got mine on ebay for like 12 bucks and the transcoder is like 50 ish now..But its a mess of cables however with the right distro it's what works best for me, most of the arcade games run at the right rez. Retropie CRT Edition. keep an eye on this guy.He sales vga or scart to component transcoder's .. https://www.ebay.com/usr/linuxbot3000?_trksid=p2047675.l2559 As far as the Retrotink goes its just the cleanest after setup, less cables and less of a mess. I've trying to get that RGB-pi working with it .