Definitely an aftermarket Motorola. Radio Shack and the like used to offer kits to retrofit most cars with an AM radio cheaper than the factory could. So, if the car had A/C, I too would speculate the original owner was trying to offset the cost of the A/C.

A buddy of mine had a '68 Charger R/T that only carried two options:
AM/8-track stereo and the Tach w/ Clock.

No Console, No Stripe, Manual Steering & Drum Brakes.

Whomever ordered the car originally just wanted to monitor their engine speed and rock out to their Blue Cheer 8-track.