I've also had this problem and it was the aerial pre-amp under the fin.
Get under the headlining and release the fin bolt ( 22mm).
{take off the weather seal and the plastic D pillar covers (with great care) asnd gently get enough space under the headlining to see the fin securing nut. It was little more than finger tight in my case so easy to get out with a socket and fingers]
The pre-amp is behind a metal plate.
I found the connections to the pre-amp from the aerial were cracked and needed re-soldering.
All is now perfect.
