Value and VIN location on 73 Duster with fold down rear seat

The vin should be stamped somewhere near the drivers side speaker hole regardless of having fold down seat or not, you'll have to remove the cardboard package tray cover to see the number, it's usually not deep enough to see from underneath. Your probably going to come across a number/letter combo that is embossed into the panel, this is just a part number used for that piece of sheet metal as the body is being assembled. The VIN stamping wont have raised letters/numbers.

Fold down back seat cars don't have provisions for speakers like a non fold down back seat car....
It is located on the driver side of the package tray and should be able to see from underneath (I could on my dart and non fold down backseat duster) but if not you will have to remove 2 trim buttons on both sides of the top of the tray cover. Then you can remove the cover and look underneath it on the driver side.