Would you be better off installing resistors in line with the indicators on the trailer?

It's not the car's fault - it's the trailer - so better off fixing the problem at the source.

Something like this:

LED Load Resistor 12V Twin Pack Bullbar Lights Front Rear Indicators | eBay

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2005 3.0d Sport - Sapphire Black
Engine : Bluefin Superchip Tune,
Wheels & Sus: Style 168s (10.5s all round), H&R RDA Spacers, Bilstein B12, Eibach 40mm drop, Whiteline RSB
Exterior: Le Mans flares, 4.6is kit F&R
Interior: Dynavin N6, Blackvue DR650 dash cams

My Build Thread: http://www.xoutpost.com/bmw-sav-foru...run-sheet.html