It is heat rope, both of mine are the standard placement belly heat, which is an inch or so in from the back of the tub. Seems to work fine for me, the corns hang out in the back when they want the heat.
As for the probe placement, it's not "outside" the rack, just outside the tubs. I have it on the 2nd shelf down,but directly on the actual heat rope, taped into the groove on the SIDE panel where the heat rope is threaded through to the next level.