Motorola 3watt AMPS Dual Glass Mount Antennas

> What was the reason for several of Motorola's "old" 3 watt car phone
> systems to have two glass-mount antennas? Was one TX and one RX?

One was on TX, and both was on RX.

It is diversity receive.
Just like the cellular towers use.
You will notice that there is usually
three antennas on each side of a cell tower.
The center one is on TX
The outside ones are on RX.

One antenna would have problems with dead spots
as a person moves, or drives, because of multi path fading.
But two antennas in different locations will very rarely fall in a
signal null at the same time.
so
The receiver automatically feeds through the strongest signal
from one of the two antennas.
If one fades for a second, it picks a signal of the one getting
the strongest reception.

And some moto 3w car phones had that feature.
You could turn it off, or on, depending on if you
had the second antenna installed.
It mainly helps the fluttering you sometimes get while
driving down the highway.