Might have been for one of these since its in the same proximity as the cigarette lighter/charging port...
https://www.ebay.com/itm/California-....c100005.m1851
The phone would have cost twice as much as the truck !
http://mashable.com/2014/03/13/first.../#I.6dh8VpNsq6
In the early 1980's I did a lot of private aviation work and a few jet owners had a "mobile phone" about the size and weight of a truck battery with a dial and handset like a home phone of the day ,