Can you add a heater/ exhaust fan unit?

Why is it cold? What is in the walls in terms of insulation, what is the floor - what is under the floor, is there a window (double panes or storm on in winter)

There are many solutions to your dilemma...some are less expensive, some are more. Generally the number of forced air registers in a room is determined by the room size. sometimes this rather delicate balance gets messed with in construction and the room is enlarged without a corresponding addition to the heating and cooling. Or...you don't have adequate insulation, windows, ducting or even air circulation. think of your bathroom like a balloon...you can only push so much air in without letting some out to "change" and become warm. So the solutions....Insulate, add storm windows or thermopane replacement windows, look into adding another register or an in wall electric heater to boost the temps and locate a thermostat in the bathroom..or tear up the floor and install an electric underfloor heat mat. Toasty heat at your feet!

If you are looking for an easy fix, and warming towel rack would be my suggestion. http://www.bedbathandbeyond.com/product.asp?sku=17738445&utm_source=google&utm_medium=cpc&device=c&network=g&matchtype=&gclid=CImav9vnprgCFWcOOgod4C8A7Q