If you haven't already separated the chassis from the body, I took one apart 25 years ago (stupid me).  Anyway, I think I pried it with a small flat head screwdriver at the rear, slightly bending the chassis out.  If I remember, there are two tabs on the chassis that fit in the body/rear bumper.  I'm sure you know already how careful you have to be with that JoHan plastic, so brittle.