Thought I should share the wealth as i have SOLVED the terrible FM Ariel signal on my 159 SW.
Bought the car and didnt test the radio
CD worked.. I could only get Radio1 and Radio4 and even the signal on those was hit and miss.
Tried removeing the ariel and cleaning the thread which did improve the situation but it wasnt good enough.
I read somewhere that it could well be a corroded ariel base (which houses an FM amplifier, along with the GPS and Phone antenna.
Firstly it isnt as hard to get to as you may think. From inside the rear of the car the headlineing has an extra panel, held with 4 clips (i only broke one getting it off) They are barbed clips that fit straight into plastic lugs monted into the roof. Gentle even pressure from hands and flat blade screwdriver will get them out. This removes the panel. Then your faced with 4 round pop studs holding the foam/composite headliner. ease them out a bit with a flat screwdriver then with needle nose pliers you can pull them out. Then there is two philips screws either side, remove these as it allows you to drop the head liner for the entire width of the car, you do have to be careful to not crease it when your arms are in there. I wedged it down either side to give more room. Also flipping the rubber boot seal out of the way helps with freedom of movement on the sides.
Id reccommend long sleeves, i came up in a rash from the insulation. iphone photos are very handy for seeing what is impossible to see
The ariel base is right by the rear of headliner and is mounted by a wopping 22mm bolt. I managed to get it off with a full size socket set. It has 3 wires, each has a clip off connector that is held to the roof, you can feel around to get them off.
Once the ariel is base is removed, you will probably see its very heavily corroded. There are 6 small phillips screws, im my case they are heavily corroded, I soaked in WD40 and got them out successfully. Once plastic case was removed, I chipped all the corrosion off the aluminum casing but the main issue was the small sprung tab that reaches up to the brass collar that the ariel tip screws into it touches with srpung pressure, it was coated it white corrosion, limescale im guessing? either way chipped it all off and cleaned up the circuit board too, see the photo's.
Then all back together and she works brilliant. I did add a small ring of clear silicone under the rubber seal of the ariel mount. Hopefully this will prevent this happening again for another 5years I hope.
Not to pleased that at 3.5 years old it was in this bad shape but the ariel bolt wasnt very tight so I guess it could lift and get water in it under speed.
Good luck with fixing yours, I hope this info helps you because I couldnt find anything on the subject.