you can also leave it and run a new one through the upper channels of the roof, keep the wiring out of the moisture that can be found in heater channels, I did this with mine and you can do it fairly easily with a fish tape, which I think you could rent from home depot.
if you decide to go with silas idea of securing the 2 harnesses together it is advisable to tie a piece of string to the end you will pull through, that way if it snags you can back it out, or in the event the 2 come apart you can still use the string to pull through..