I finished the game and now I'm just roaming around and just taking over territories.

I tried to taunt civilians to get the trophy(I think you need to taunt 10 people or something) but when I checked my stat, it said I have taunted 0 people.

Found that very strange but whatever, maybe I'm doing it wrong so I move on to the next trophy. Customize 10 cars.

I started collecting and cars and just to make things to easier, I put nitro on each car and just move on. After about 12 cars, no trophy.

Maybe putting nitro wasn't enough. So I upgraded torque for 10 cars. Again, nothing.

Am I doing something wrong or is there something wrong with the game?

The only thing I can think of is you're supposed to do it during the game, not after you beat the game. But I don't know.

Does anybody know what's going on?

I think it's supposed to be gang members I've taunted about seventy of them but it still shows zero for me too. I don't get it.

You need to taunt gang members while you have no notoriety for it to count towards the trophy. As for the vehicle trophy, maybe try cosmetic changes? That's how I got it. I don't know.