In my racks, I wired each shelf with one foot of 11" Flexwatt. I cut 6" sections of extension cord and split the wires apart with the ends stripped. I then solder one wire to each side of the tape. On the upper most shelf, I solder an extension cord long enough to go down the back of the rack and have enough length to plug into a thermostat/dimmer. I then strip the extension cord (about a half inch section) at each shelf. Then I take the wires from the other 'heat pads' and wire each one to the separated, stripped wires of the full extension cord. Of course all connections are insulated with electrical tape and/or silicone. If you need pics, just let me know, and I can try to get some posted in a day or two.
Dennis