You are allowed to use car mods, Emergency lights which are ELM, You can mod you sirens and sounds of your guns and the looks of your guns and other sounds in the game. Just as long as the mod that you have installed doesn't give you an advantage over the other player.
a Mod thats not allowed are extended maps on your screen that other players can't see. What I mean by this, say you had an extended map. You go up a ramp that you see that leads to the extended map, The player who is around wont be seeing you driving up a ramp, All they would see is you making your car float and think that your hacking. I know this because I had this done to me, I was a cop chasing a suspect and he went up an invisible ramp that lead to the extended map. And I saw his car just floating up and up and up. So I reported it thinking he was hacking But he just had something I couldn't see, which was the map that he had and nobody else did.
So yeah anything that doesn't give you an advantage is allowed.
