take off your center console (the armreast) and tighten the cables. its one of those routine maintenance things... i think every 60k or something like that. you gotta jack up the rear end so you can spin the tires by hand. you should be able to feel it starting to grab by the 1st click and then it should be fully engaged by the 6-10th click from what i remember.
i did it not too long ago. pretty simple if you get it right the 1st time and dont have to re do it later. but eventually it turned out not to be the problem for me. now i need new rear calipers because the ebrake part froze up. but more then likely you just need to tighten the cables