// at top of the script
new specialcar;

//under OnGameModeInit
specialcar = AddStaticVehicle(blahblah..);

// at OnPlayerEnterVehicle

new skin = GetPlayerSkin(playerid);
new carid = GetPlayerVehicleID(playerid, vehicleid);


if(carid == specialcar && skin == skinidhere)
{
    //got right skin and is in that car, hes allowed to enter
}
else
{
    // if the player dont have the skin, remove player from vehicle