yes they come pre-wired with 3" flexwatt heat tape on the bottom. the way i have my rack set up is like so:

I have a thermostat with a temp probe and an electrical output.

You tape the temp probe straight onto the flexwatt heat tape, outside the tub. you don't want your snake moving the probe.

Then you plug the rack into the thermostat. As the temp probe senses temp changes, it will turn the heat tape on and off to adjust to the temps.

now, you have to keep an eye on the inside of your tubs, so most people get a digital thermometer (acurite works well) and put that probe on the inside of your tub, straight on the plastic where your snakes belly will be.

Set the Thermostat at 93 to begin with. If the thermometer (inside the tub) reads 89, then bump the thermostat up to 94. Small increments will eventually get the inside of your tub to around 93 for the warm side.

this is just a brief rundown of how I did my rack. there are lots of other ways to do it, but i think the way I've done mine is the way I read a lot of others do theirs.