| It IS down to the wattage,The Kellermanns are Halogen lamps and are only rated at 5W(even though they are bright!),your relay requires a parallel circuit of 2 X 21w,you can SOMETIMES get away lower wattage.The relay is seeing exactly the same as when a bulb blows,ie too little wattage.By putting two resistors in the circuit of 10W each you are giving the relay what it needs to flash at the correct speed.Make sense?It does to me!!Roughly it should be wired like this: | 
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