Morning Paul
Welcome to the Forum
My bet would be the Diversity antenna has suffered water ingress (common at around your cars age) or the loom (left hand side) where the boot hinge is has a crack in one or more of the wires (less common). My money would be on the Diversity antenna try holding the key on top of the rear spoiler that sometimes works
Remember if you change the antenna your keys will need to be re-programmed to the car. Use www.realoem.com to get the correct part number for your car then use Google to search for the part. If your antenna is not in to bad a state you could try repair specialists like Cartronix or ECU testing to see if they can fix it.
Changed a few of these including my brother inlaws 😁
Good Luck
Dave