I have trophies showing for all of the main races, all of the maker races, and I've got the red-circle in all of the "arcade mode" races, but I'm still at only 96.86%. What do I need to do for the last 3.14%?
if i recall correctly you need to complete all of the manufacturer events. Check through them all - particularly vauxhall/opel (as this is technically two manufacturers that are interchanged based on your chosen language in the EU version, it just displays opel elsewhere)
If you have all the Vauxhall events complete, try to get an "opel" and do them again (which would require you to play in a different language to acquire)
Not certain if this is 100% accurate but I remember that the vauxhall opel thing caused problems for people in the past, maybe this is the issue you have now.