I just had a thought (no snide remarks please) - the people who have chimed in on this thread and said LEDs work in their tail lights in ERAs, have the round tail light option which I believe uses 4 bulbs instead of one being a reflector. Kevens2 has round lights, I know Patrick does, I think Large Arbor does. I have the rectangular tail lights with one bulb each.
Possibly, it takes the 4 LED lights together with the front ones to create the resistence properties necessary for everything to work. Without my parking lights on there just isn't enough load in the system and something is going wrong. When I turn on my parking lights the extra load on the system brings everything on and it works fine.
Is there anyone out there who hasn't given up on this thread already, with rectangular lights and had sucess with LEDs in the tail lights.