I have not had time to get back into the tail lamp issue yet...
I agree with you Andy, and am aware that LED's are polarity sensitive.
My question was that it had been my experience that the center pin/pins on bulbs was normally the +... and these marker lamp fixtures were not like that.
So here is my protype solution... to convert the lamp to + on the center pin.
I removed the ground strap(silver) contact and cut it off within the recessed area.
I then drilled thru the center and inserted a bolt...
Pic of modified lamp and original front and back.
The red wire will now supply + power to teh bulb pin, and the original black wire will be ground.
I need to do some detail work on the bolt, nut and connection yet to make sure they are good and safe, but I think this will work.