Nope, you didn't do something wrong...... mine was doing the same thing, so I sectioned the rollcage and reworked the interior tin. Still not entirely satisfied (which is probably why I haven't finished it yet), but it's better than it was.
I enjoy reading the Model Car thread each week, and thought I would add some pictures of my older builds. This is the oldest car in my collection...I built it in 1974 when I was fourteen. AMT's Fiat model with sewing thread plug wires, injector scoop made from a modified pen cap. The rear tires and wheels were added about twenty years ago from Monagram kit.
Again it is AMT Fiat kit... I built this one in 1995, it is not detailed with wires, and fuel lines. This car was built as mock up to the real altered I built. If you look at my avatar you can see the real car.