[dudermagee]

Aftermarket parts do count towards the value of a car if someone else hits you.
If its your fault then they don't unless
A: you actually claimed it before hand and you are paying for extra insurance
B: your insurance company is really nice for some reason

They will give you shit for stuff you have but don't have receipts for, however that just means you'll have to do research and find comparible parts and send them (printouts, shortcuts, ETC.)over to them.
DO NOT LET THEM FUCK WITH YOU!
You are not the one at fault and they OWE YOU, not the other way around, if they keep dicking with you tell them you will get in touch with your insurance company and their lawyers will be contacting them shortly (which they will as long as you have full coverage).