I see. In this case, the user's back will be acting as a pretty noticeable heat sink, but it will depend on the thickness and thermal conductivity of the material between the tape and the person's back. Body temperature is about 37C, but the skin surface temperature may be several degrees less.
It may make sense in terms of efficiency to have slightly thinner, more thermally conductive material on the side that will be facing the user and thicker, more insulating material facing away, so that more of the heat is transferred to the user instead of into the air.
The device will probably need some kind of temperature-controlled cutoff so that it doesn't overheat or get uncomfortably hot if left on in open air, such as a high power MOSFET for switching the heating element on or off. There are a variety of ways that one could read a temperature sensor and switch the MOSFET on and off, ranging from a simple analog comparator (possibly with hysteresis) to a programmable microcontroller. I would recommend using binary switching or PWM-control rather than linear control for the MOSFET to reduce the amount of heat it dissipates.
A lot of this is predicated on the operating characteristics of the tape. Until you know what its actual resistance is, it is hard to be sure what voltage/current it will need to run at and how you will need to configure the battery. It may make sense to get a sample of the tape and measure it-- or even do some experiments with a bench power supply-- before designing the system to control it.