Shirley its down to the watage of the bulbs......
I'm no electrician, but the system need 2x 10 watt bulbs operating in series to operate correctly.
I recently fitted LED's (rated at 0 watts) to the front mirrors and uprated the rear bulbs to 23 watt and the system works fine.
Most aftermarket indicators seem to be fitted with 23 watt bulbs, two of these units front and rear would certainly stop the flasher unit working correctly(46 watt on a 20watt system), drop back to 10 watt bulbs and they work fine.
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