So, I did some serious digging and decided to try an aerial adaptor - one that sends power up the coaxial cable (a Connects2 CT27AA05, but any of the same sort would do) - and like other posters on other threads I have had 100% success with my Pioneer unit
In short, using the adaptor has improved ok reception and given pretty good AM where before there was none (white static and carrier wave only).
Now here's the science bit... (slightly OT because this is for the Brera)
Unknown to Alfa dealers, ICE dealers and most online resource, the aerial in the Brera (and presumably the 159) has an amplifier built-in behind the C-pillar. This requires switching on by a +12V low-current signal up the aerial coax, which the OEM headunit provides, but an aftermarket unit does not. The amplifier receives power and earth seperately from this switching signal.
The adaptor (I am not calling it a booster or an amplifier, because it's not) contains simple gubbins for connecting the voltage and a 100uF capacitor, to prevent the 12V going down the coax and reaching the sensitive receiver in the stereo. The blue wire is connected to the antenna accessory on the headunit.
Hope this helps someone!
Here's an extract from the Brera eLearn manual (note the two different AM and FM aerials):
The structure of the aerials consists of:
- AM aerial comprising serigraphy on the right rear side window;
- FM aerial which uses the heater elements of the heated rear windscreen;
- telephone (GSM) and navigation (GPS) aerial on the roof;
- navigation only aerial (GPS) concealed in the roof trim.
The AM/FM system comprises an AM and FM amplifier where the AM signal arrivs from the serigraphy in the right rear side window via a unipolar cable whilst the FM signal arrives from the heated rear windscreen filaments via a coaxial cable with screening earthed at the tailgate right near the connection to the serigraphy.
The cable that carries the signal to the radio module leaves the AM/FM amplifier.
1 - amplifier
2 - AM signal connection from side window serigraphy
3 - FM signal connection from the heated rear windscreen filaments