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UNDER OnPlayerConnect: 
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SetTimerEx("AutoRepair",500,true,"d",playerid);
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Put this at the end of the script:
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forward AutoRepair(playerid);
public AutoRepair(playerid)
{
    if (IsPlayerInAnyVehicle(playerid))
    {
        new Float:health,
              vehicleid;
        vehicleid = GetPlayerVehicleID(playerid);
        GetVehicleHealth(vehicleid, health);
	if (health < 250)
        {
            SetVehicleHealth(vehicleid,1000);
	    GameTextForPlayer(playerid,"~n~~n~~g~Your vehicle has automatically been repaired!",1000,3);
	}
    }
}
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