Mark,
Like I said in the previous post, use a tone generator to locate the wires needed. If you are not familuar with the use of a tone generator, just Google the phrase "tone generator tool" and you will find many options from $300 Fluke tools to $25 Harbor Freight tools, or you can just borrow one from a friend. For what you are doing, the $25 units should do just fine, and you can find operating instructions online.
Gerald