[HELP]Adding custom visual customizations to cars

[edoo1]

So I downloaded some modded cars that replace default GTA: SA cars but they're all stock and don't have customization options. I have no idea how to add those visual customization options. Is there a folder or anything special ? I would appreciate any kind of help.

Edited October 19, 2018 by edoo1

[rfmx]

If these car models you have downloaded don't have custom parts in the model file your only option is to add them by modelling them yourself.

[edoo1]

I already have the custom parts. But I have no idea where to install them. and how can I change the wheels in the tuning shop(every wheel is the same)?