Deja vu all over again.  We just had a thread on this in the past week
or so.

Easiest thing to do is mount it on the dash, just behind the windshield
(inside the car like they tell you NOT to do).  It works perfectly fine
~ 95% of the time.

You can either put a little piece of Velcro on it and the dash or, as I
did, mount a very small piece of sheet metal* (talking maybe 3/8" x 1")
on the dash with PSA and let the magnetic antenna cling to that.

*I'd bought one of those little magnet holders for my cellular phone and
this came with a small black piece of metal with PSA on one side.  Its
purpose was to attach to the back of the cellular phone in case it was
not magnetic.  Didn't need it for the cell phone but it worked just fine
to stabilize my little XM radio puck on the dash of my '84 convertible.

There is steel in the halo, dunno if your magnet is strong enough to
hold the antenna tho. Another possible is doing something with the fuel
door.

Suggest you dig back to the earlier thread. "Sirius" in the subject line
-- but it dealt with both systems on C4s.  That thread showed that
results vary with year built. I've had my Roady2 antenna up to 130mph on
the C4 and the magnet holds it fine--neat installation, very little
shows.  Bottom line is that outside antenna works better in northern
latitudes and has fewer hiccups around tall buildings and in and out of
tunnels, overpasses etc.
--
PJ

As everyone else has mentioned, center of the dashboard as close to the
windshield as possible works great in my 96.  I only lose signal when you
would no matter where you mount it such as in a tunnel etc.
Rob
78 & 96 Coupes, Red of course