right mate... I will be totally honest here as it will help you no end!

the wheel arch - and the whole car looks like it has a "smooth added, like you would on an organic material....

maybe try nubs around the arch?

or if not ..get nicer shpes with boolens?

the back area from this angle does look quite ok...

have you seen mike mckinleys poly car tut on his site? It may help if you prefer sticking to polys... but dont think for one minute that you have wasted time by my comments as it is all practise mate!!!

well done