I was looking at the factory manuals for my Dyna, and it has the most beautiful fold out color wiring diagrams you could ask for along with descriptions of what each wire does and what color it is.
I am sure your manual will have these pull out charts also. What we are looking for is the pull out chart that shows the multi pin connector that plugs directly into the back of your radio. It will show all the wires for the speakers, power wires, ground wires, wires for back lighting of the dials, maybe some wires to work with you cell phone and who knows what else. Another name for this is a "Pin Out Chart" That is the picture you really need to have or should have to help you with this problem.
Once you have that along with a meter you will be ready to do some real damage.
It will all make a lot more sense when you see your pull out charts.
I must say HD did a Great job with the wiring diagrams in their service manuals. At least for my bike.
Hoop!