[Improvement™]

pawn Code:

//Somewhere on top of your script
new bool:VehicleGodmode[MAX_PLAYERS];

//Somewhere between all the Callbacks (public's)
COMMAND:godcar(playerid, cmdtext)
{
    if(!VehicleGodmode) VehicleGodmode = true;
    if(VehicleGodmode) VehicleGodmode = false;
}

//under "OnPlayerUpdate(playerid)" Callback (public)
if(VehicleGodmode) return RepairVehicle(GetPlayerVehicleID(playerid));

This should completly work.