Try it like this, assuming inches.19.5" width x 8.5" height x ??.?" depth.
He also needs the measurement from the front of the seat to the rear wall of the cab.
The depth will tell him how tall of a sub you can use. The height will tell him the volume of the enclosure, so he can match the proper sub to it.
It would also help a bit to tell what kind of music you listen to. As well as what kind of sound you want. Nice clear punch or pounding loudness? In a lot of cases there are a whole lot better performing and sounding subs than are popular in your area. Since you said memphis most likely you are more interested in loudness than clarity.