First off, you'll either need a 2-stage temperature controller (preferred) or a single-stage temp controller that is switched over to heating mode.
Next, Williams Brewing sells a stick-on sheet that heats up. It's basically a really thin space heater. Called "the brewer's edge space heater." I use one for my fermentor in my basement in the winter months, but it would work nicely for a cramped kegerator / keezer. It will keep a small space up to 20° warmer than a surrounding area.