In the right rear cargo compartment there is what looks like a stereo speaker. It is about the size and shape of a 10 inch stereo speaker.

There is no corresponding "speaker" on the left side, which leads me to believe it is not for the stereo at all.

Any ideas on what this might be?

Subwoofer.

Ah, good. I had thought that might be under the front seat or somewhere... well, actually I suppose it IS somewhere - in the right rear cargo area!

It is an eight inch subwoofer with (I think) 150 rms rating.

2005 AWD Escape Hybrid
Best Interstate tank trip MPG 39.02 (scangauge II) for 402 miles on I-70, 10.3 gallons used over mostly flat terrain. Aug. 2007

Best Interstate tank trip E30 MPG 34.6 for 271 miles along I-80 in Indiana and Ohio. May 2008

Best multiple road tank trip E30 MPG 36.2 for 202 miles in Southwestern Pa. July 2008.