If I understand the description you gave properly these antennas are for the
XM satellite radio receivers. On semi tractors, look for a whip mounted on
a rear view mirror with an inverted cone at the top of the whip that tapers
for about 6 inches or so. The wide part of the cone isn't a lot larger than
the supporting whip but it is noticeable.

Normally on a hardtop car the antenna is placed near the rear of the roof.
If it is a convertible then they will likely mount it on the trunk lid at
the back of the vehicle.