What year is your 4Runner? It could be a number of things, the most likely suspects are the cable or the parking brake bellcranks being seized.
I just finished overhauling the parking brake system on my friends Tacoma last night, and the problem with his was a seized bellcrank. We just removed it, took it apart, cleaned off the rust, put it back together, greased it up and re-installed it. Works like new.
If it's the cable that is seized, I don't know how successfull you would be trying to free it up and get it working properly again. I don't have any experience with that, but you may be better off just replacing it.
Your first step is to find out where it is seized.