Fair enough then, guess you don't get away with the cheap/easy fix. I too was certain it was the suspension - the noise very cleary originated at the drivers' side rear wheelwell. I'm assuming the noise is the outside pivot on the folding seat.
The rear suspension of the 124 is certainly known for wearing out bushings. The thrust arm is particularly prone - it has been upgraded several times over the life of the five link suspension. If you've never had the thrust arms replaced, this is the place to start looking. You can use the search feature to find past discussions of the issues.