Typically the wrap is wound around the outside of the PVC or copper drain pipe. It can be hard wired into the defrost timer so that it runs all the time except in defrost, or you could run the tape to a receptacle mounted on the box so that it uns all of the time.
How is the pitch on the drain? For a W.I. Freezer I like to see a strong pitch of around 4" per foot. Also insulate the drain with armaflex.
You want to have as short of a drain run as possible inside the freezer. Make sure to trap the drain on the outside of the box.
It is possible that the Defrost Time Clock is mounted in the panel with the other controls.