Poor Radio Reception

Good evening. I know this seems like a minor problem to be adding to this forum but I am a bit stuck and starting to get a little frustrated with the problem.
I have a Ford Mondeo MK3 Estate. I have replaced the factory fit radio with a Sony MEX-BT3100U. The radio reception in the car has always been a bit hit and miss but just recently it had got worse. I will try and explain what I have done to try and rectify the problem.
Replaced the antenna and checked that it is earthed well.
Replaced the antenna cable from the antenna to the stereo (checked for good earth and short from central probe to the earth shield)
The antenna has a 12v supply for a built in amplifier and I have also tried fitting an inline amplifier.

The radio reception is better when the antenna cable is run direct from the antenna to the stereo without routing it behind trims and the dash. As soon as the cable is run down the A pillar and behind the dash the reception drops drastically.

My next port of call would be to replace the radio unit but I wondered if there is any further tests I can run with or without a multimeter.

I hope someone can help!!! PLEASE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1