My 87 wagon had the same problem. It turned out to be the interlock system needed lubrication. It was just plain stuck from lack of use. If I remember correctly, I took out the seat bottom, then worked the interlock from top and bottom by hand. There is a small lever that lifts up when the seat bottom is lifted. That part was stuck. A little WD-40, and presto. I'll look at it in the morning to be sure that I'm remembering all the details.