1999 4964ex and LED lights

Have you tried putting regular incandescent back in the circuits? The ECC is not made to work with the super low resistance of led lights. The turn circuit works off of resistance to flash. The 4 ways work off of "timing" so to say. The reason the LEDs might have worked for a while could have been resistance in the wiring from corrosion or such.

 

Try plugging in incandescents into the trailer plug and see what happens.

 

If you just switched from regular old bulbs/lights to LED you will have to install a electronic flasher similiar to this one (but yours may have different plugs)
http://www.grote.com/products/44891-variable-load-electronic-led-flasher-flasher/

 

Yes also make load resistors for converting from regular bulbs to LEDs. Auto part stores have them and they are wired to the hot side of the LED and to ground.