I think i had a similar level of training (community first aid with cpr) and i think all i'm really qualified to do is ABC. For car wrecks (and in my case, bike wrecks), the place where limited training could be the most help is in preventing someone (or yourself) from bleeding out which may happen in minutes. I've taken to carrying around a couple of those trauma bandages with integral elastic wrap, before that i had a bunch of 4x4s and surgical pads and ace elastic wrap which is basically the same thing.