I would guess that the signal lamps on the hitch light are low wattage lamps and don't provide a large enough load to "fool" the LCM into thinking the trailer lamps are OK. You could try clipping a 12v 21w lamp across one of the signal lamps and see if that changes the flash rate on that side.. then you'll have an answer. If this works, then you'll want to add a couple of resistors to each side to increase the load.

What is the wattage rating of the lamps on the hitch lamp? I'm guessing they are only 5 or 10w.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)