I was just wondering if it's possible to change only the left amber light on the back to be red, while keeping the right-side light amber. If this is possible and somebody could let me know which textures to edit that would be very helpful!

You can change the colours of lightbars by editing the textures found inside your vehicles ytd file. There might be multiple files called something like "emissives", "emergency", "lights", etc.
Sometimes you can already see which part of the lighting setup they are for, for example, the light bar, rear lights. However, sometimes you can only guess which file alters which lights.
Maybe that gives you a starting point in solving your request.