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There is a large piece of loosely supported plastic that sits behind the rear seat in my trunk. What purpose does this serve, and is it supposed to be attached to anything? I do have the rear seat pass through.

Every Bonneville of this vintage has the rear seat pass-through. There is a large plastic panel the spans the width of the rear seat. The panel you are talking about is probably that piece of trim and has just become dislodged. A pic would help identify it for sure. In my 2002 SE the panel was a slightly different style than the 2000. Not sure which one the 2001 has.

ddalder wrote:Every Bonneville of this vintage has the rear seat pass-through. There is a large plastic panel the spans the width of the rear seat. The panel you are talking about is probably that piece of trim and has just become dislodged. A pic would help identify it for sure. In my 2002 SE the panel was a slightly different style than the 2000. Not sure which one the 2001 has.

Pic on it's way.

The reason I ask is that I get a thumping sound coming from the back of the car when I go over certain bumps at certain speeds. I thought that it was that rear deck problem, but that piece that the speakers sits in is tight as a drum.

This big piece of plastic, however, is flapping around without any connection at all, except for 2 points where it's attached to the top of the trunk.

Question: Does that plastic piece where the speakers sit act as a resonator by any chance? Can I fill it with insulation to stop that thumping sound?

I had that big plastic piece removed for a wrong time, put it back now with some insulating foam to stop buzzing. It made this goofy chirping sound, which is why I originally removed it.

In my experience, if your clips aren't broken, they require a bit of force to get back in solidly.

kstills wrote:

ddalder wrote:Every Bonneville of this vintage has the rear seat pass-through. There is a large plastic panel the spans the width of the rear seat. The panel you are talking about is probably that piece of trim and has just become dislodged. A pic would help identify it for sure. In my 2002 SE the panel was a slightly different style than the 2000. Not sure which one the 2001 has.

Pic on it's way.

The reason I ask is that I get a thumping sound coming from the back of the car when I go over certain bumps at certain speeds. I thought that it was that rear deck problem, but that piece that the speakers sits in is tight as a drum.

This big piece of plastic, however, is flapping around without any connection at all, except for 2 points where it's attached to the top of the trunk.

Question: Does that plastic piece where the speakers sit act as a resonator by any chance? Can I fill it with insulation to stop that thumping sound?

I would suspect that a thumping sound would more likely be coming from the rear strut mount, strut or a rear suspension component such as a control arm bushing or rear frame bushing.
You can have someone else drive, while you sit in the back seat to listen for the noise and at least you will be able to narrow down which side is making the noise.

I dont have that panel in mine and I have the same thumping you describe. I sure would like to figure out what it is too. Replaced rear struts a year or so ago so mine isnt that.