Are you sure it's the top? Carpet with mildew is a nasty smell. Years ago my son spilled a Slurpee on the carpet in the back seat area of my truck but didn't say anything. I tell you what, my truck smelled nasty after awhile. Almost to the point of unable to drive it. Maybe something spilled or water leaked onto carpet somewhere? Under carpet is some jute style insulation is some places, and that stuff has a chemical smell when wet (we have hundreds of rolls of it here at work). I've never had a convertible Mustang, but I have had other cars with soft tops, and I've never had one smell like you're describing.
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