I too had this problem. I traced it to a link which had become disconnected at the bottom of the seat back.
If you haven't fixed it yet post again & I will have a look at mine again & give you more detail.
90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)