As a ham for 25 years I have installed many mobile radios.  In the 74 bus I
had the antenna mounted on the roof which is usually the desired location.
 The antenna took a lot of flexing to getit into the garage and would break
about once a year. I did not mount the antenna this way in the 84 vanagon
because the dome light was not in the center of the roof. I needed this
opening  for access. The first antenna was gutter mounted near the rear air
intakes and I snaked the coax down the air intake opening through the
undercarriage and into the cab area.  This never did work too well.  I
finally mounted a second antenna in the same spot as the AM/FM antenna but on
the passenger side.  Required cutting several holes buts works like a champ.
 The radio mounts nicely on the plastic panel covering the heater outlets.  I
was not too sure it would be stong enough but its working fine.  Direct power
wires to the battery is also recommended to prevent ignition/alternator
noise.
Bob