As I understand it the indicator circuit requires the resistance of the two bulbs plus the instrument repeater as a total load to make the old type indicator flasher unit (a fairly crude but simply effective device) flash at the correct or any rate at all and so function as it should. Leds just don't generate the circuit load to achieve this so need something built into the circuit to simulate the original load but as far as I can see this is not applicable to a tail and stop light as there is no minimum resistance required to make the circuit operate.
If the product is listed as 12v supply then you should just be able to put it onto a car battery to check it as it should need nothing special to run it.
