I just went through this with the steering cable that came with my current project boat. I removed the cable from both ends. Took a plastic bag and cut one of the bottom corners off where it made a hole just barely large enough for the cable housing to fit through. I pushed the cable through the hole until the stationary outer portion of the cable was just entering the hole by about 1/2". I wrapped the bag to the outer cable with some good tape to form a tight seal around the outer housing. Then I hung that end of the cable from the rafters in my storage building. I supported the upper edge of the bag so it wouldn't drop too low, then I put a few ounces of penetrating oil inside the bag and waited for the oil to do it's job. I suspended the other end of the cable just slightly lower than the oil so that any oil making it's way down the cable would stay inside the cable and not run out on the floor.
After a few days of this, I reinstalled the cable to the steering wheel end and turned the wheel. The cable had a little resistance at first, but it began to move and with a little working of the wheel, it started to turn fine. I hug the cable one more time, but this time I put a light weight machine oil in the bag, then left the other end sitting in a catch pan. Once the oil started coming out the bottom end, I took it down and installed it.
I only did this as a temp measure to check the boat out on the lake. You should probably replace it, rather than taking a chance that it will seize up again or possibly even break. I've started converting my boat to a tiller setup and removed all the remote controls.