Any Fiat 500 with the Bose 6 speaker package has a unique bass speaker mounted under the passenger seat between the seat frame rails. It is a flat package bolted to the rails and well protected from "debris". The housing consists of two horns [folded ported bass ?] that sit flat, but are wound around the speaker chamber. The housing appears to be only 1 or 1.5 inches in depth; but, it may fit into a depression in the bottom. Worth looking at if you need more bass and are unwilling or unable to mount large speakers in your tire well. I can not tell from looking at the top side the depth or diameter of the speaker. It sounded great in the Fiat I rented over the weekend.
G