So I wired in a universal trailer wiring harness as per http://www.discoweb.org/trailerwire/index.htm and when I hook up to my trailer, on which the lights all work and don't cause this issue in other vehicles that use the trailer, the Rover's lights and trailer's lights all flash double time, either when on as a turn signal or 4 way (hazard).

Unhook the trailer, and the Rover's lights work fine and flash at their normal pace.

Interestingly, if I have the 4 ways or turn signals on with the trailer attached, and depress the brake pedal, the pace reverts to normal speed. Release the brake pedal and back to double time.

Thoughts?

 

Are the trailer lamps incandescent or LED ?

LED bulbs (much lower current) can confuse the on-board compfuser.

 

Yes, LED. Can a heavy duty flasher relay be installed?
Or am I just relegated to use non-led lights?

 

Go to superbrightleds.com

They sell load resisters that you attach inline on the LED lights.

They also sell flasher relays, but I can't tell if they will work on a rover.

If you find one that works, please post the part number.

 

Sorry for the delay.
A heavy duty flasher will do nothing. The on-board compfuser senses the little current from LED bulbs and thinks a bulb is burned and warns the driver with double frequency. Normal behavior.
An in-line series resistor will do nothing.
A resistor in parallel with the LEDs will correct the problem but it is a waste of heat and complexity and reliability, and counteracts the convenience of LEDs.
I would stay happy with the fast blinking when trailering and ignore correcting it.

 

My fix was to install standard bulbed trailer lights while I was redoing my trailer.
$22 on sale & 20% discount at Harbor Freight. Can't beat a full set of lights and wiring harness for less than $19!