I decided to whip out the old camera and post a few pictures of some spare seats for you, even though they're kinda rusty and not very pretty. The photos are all taken such that the forward edge of the seat is at the bottom of the photo. In this photo the seat rails are locked.

In this photo, you can see how the main lock release lever pulls on a secondary release lever on the other rail. If your cable is broken between the two levers, you should be able to reach under and release the secondary lever by hand.

Here's a close-up of the secondary lever to give you a better view. Hope this helps.