Posting this as an FYI, coz I searched everywhere for the best and cost-effective way to do but struggled finding the info for my Ford Ranger PX, so hopefully helps someone else out once this page gets indexed by Google ;-)
I bought a Jetski and the trailer that it came on had LED lights. I got the hyper-flashing issue (where indicators flash at double speed) It's a common problem for many vehicles with trailers that run LED bulbs. Fixes are as follows (I'll post links so you can click to see images, but basically buy these parts almost anywhere)
1. Changing your car to an electronic relay (for example an "EP27")
2. Wire in load resistors, one for each indicator bulb, to take "load" off the LED bulbs and make them flash normally
3. Buy and use a "Genuine Ford PX Ranger LED Light Trailer Adaptor/Plug/Loom Resistor Focus Towbar", eg:
Option 1 was most appealing, but unfortunately this is only relevant on older (< 2011 I think) model Rangers (I also understand that Ford Ranger PX2+ were fixed to work with trailers with LEDs) Cost for EP27 around $40.
Option 3 was the easiest solution, but didn't really appeal because it was a part that could be lost or stolen quite easily. Also expensive, $200+ in NZ (or about $120 via eBay)
So I went with Option 2, also the cheapest ($10 plus $5 for a switch) however I also have a normal trailer with incandescent bulbs and apparently you shouldn't run load resistors with normal bulbs, so I wired the load resistors onto the wiring loom under the rear of the truck that goes to the plug. I also wired them via a toggle switch, so I can simply reach up and flick the switch down for LEDs (pulls load resistors into the circuit) or flick the switch up for incandescent bulbs (removes the load resistors from the circuit) Only a slight pain really as I'm bending down to put the trailer on the tow bar anyways. I wired them onto the truck itself rather than the trailer with LED lights, in case I need to tow another trailer that has LED lights, plus there isn't a great deal of room to mount the load resistors on the trailer itself. A suitable workaround for my requirements.
Main downside of load resistors is they can get pretty hot, so I mounted onto a metal surface to help draw heat away. I also mounted them high as I could so if reversing the jetski into the drink, they're still away from the water.