Probably too late now, but here is a copy of what I posted in the Factory Antenna thread in December. I believe that the headlining support panel is integral to proper earthing of the antenna.
"Having dislodged my antenna solid mount by adjusting it too forcefully a few weeks back I went through the process of removing the mirror, sun visors, "A" column finishers and screws at each end of the front headlining section, which was simple enough.After re-securing the antenna the radio reception was rubbish, so removed the radio and antenna connection at the back to check it and almost concluded that the cable was fractured, so left it to heal for a couple of weeks.
We had an electrician doing some work in our home so I asked his opinion. Reception was ok when one of us was holding the cable and earthing it.
He thought that perhaps the jagged washer was not earthing to the roof properly because (after 51 years) the paint was too thick so I sanded that back. Partly fixed the problem but reception varied.So sprayed WD40 on and sanded the antenna rods, to not much avail.
Then when cleaning the visors and headlining section I noticed a dent/impression in the mild steel headliner support panel where the screw section of the solid mount had been resting so sanded that and as they say in various aftermarket service manuals "offered it up" to the antenna. Reception immediately improved and when everything was re-installed and all the screws tightened reception is again perfect.
Conclusion: The headliner support panel is an integral part of the earthing circuit.
Could have saved a lot of fiddling and thinking time and frustration if I'd realised that earlier, so hope some-one else can benefit from this experience."