Since the switch is all the way in the train of the dress, you'll need about 4 feet of conductive thread leading to the dock. Use a fabric bias tube for a stylish way of insulating the thread.
Textile enchantress Lyne Bruning shows how to make a bias tube in this
Instructable.
With a large needle, feed the conductive thread down each bias tube... you'll need two to keep the conductive threads separate.
Follow
these instructions from a previous example to connect and assemble your magic dock. (the colors of the bias tubes are different, but the technique is the same)