forward RespawnCars();
public RespawnCars()
{
    new bool:VehUsed[MAX_VEHICLES],vehID;

    for(new playerid,g = GetMaxPlayers(); playerid != g; playerid++)
        if(IsPlayerConnected(playerid))
        {
            vehID = GetPlayerVehicleID(playerid);
            if(vehID > 0)
            {
                VehUsed[vehID] = true;
                new vehMod = GetVehicleModel(vehID);
                if(vehMod == 403 || vehMod == 514 || vehMod == 515 || vehMod == 525 || vehMod == 531 || vehMod == 572 || vehMod == 583) // Linerunner,Tanker,Roadtrain,Towtruck,Tractor,Mower,Tug
                {
                    vehID = GetVehicleTrailer(vehID);
                    if(vehID > 0)
                        VehUsed[vehID] = true;
                }
            }
        }

    for(new vehicleid = 1; vehicleid != MAX_VEHICLES; vehicleid++)
        if(!VehUsed[vehicleid] && IsValidVehicle(vehicleid))
            SetVehicleToRespawn(vehicleid);

    return;
}