Converting a 2 wire to 3 prong pig tail

Not sure if this will help, but don't forget that led lights are polarity sensitive, meaning Ground has to be on ground, otherwise it won't light.

 

just join the red and black wires together to power and the white to ground and it will work fine.
we did it like this for our long load lights all the time, if one element failed the other was there for back up.

 

It shouldn't. Three prongs are normally for things like turn signals and break lights. The third prong will normally just intensify the light output. They can also be used on dual output lights (red and amber for example). The biggest thing is insuring you have ground to ground as mentioned before. And with LED lights ground is often White but can still be black. You should be able to get the lights to turn on with just two wires. But splicing the two hot wires into one will work also provided it just intensifies the light and doesn't change its color.