I don't know how to do this,(to link to another thread - being so very old)) but if you can get into the thread below, there is some advice about poor radio reception. I am waiting fro a response from Squadrone Rosso as to whether the work he had done (rewiring form head to rear screen aerial did the trick.
Boosting the signal from the rear screen aerial will only make matters worse when your rear demister is on - all you will be doing is increasing the interference. It may help matters the rest of the time but the best fix is to fit a proper roof aerial, like the facelifted 156's have.
Basically, there is crap getting into the aerial, if you amplify the output of the aerial, you amplify the crap too. It's physics I'm afraid, and ye cannae change the laws o' Physics, Captain!
Basically, you have the aerial (built into the rear screen) sitting right next to something which apart from generating heat, generates broadband radio-frequency interference (the rearscreen heating element). The interference goes right down the aerial! If you amplify the aerial, you amplify the interference.
It's a typical Italian thing of form over function - not having a big old aerial on the roof looks great but it doesn't work because it sits right next to a big old heating element! When the rearscreen heater is turned off, the interference disappears but the RF performance of the aerial isn't that hot anyway to be honest. The best aerial is a straight one, polarized vertically, like the transmitter mast, not stuck inside a flat piece of glass.
Strange! Reception of LW (Radio 4 Test Match Special) on my radio only works reasonably if I switch the demister on! I have a post 2002 Facelift (pre 2003 Facelift) model with a "bee sting" aerial on the roof, but am convinced that is not connected to anything, as connecting a longer aerial to it makes no difference at all. It's probably there for a phone connection. If Squadrone Rosso picks up on this thread, did the re-wiring he spoke about work?
The LW aerial will be a ferrite rod aerial, actually built into the headunit - that's why it's not affected by the rearscreen heater.
Regarding the fact that the aerial worked fine until a new headunit was fitted, hmmm... sometimes an aerial needs to be tuned to the headunit - sounds crazy but the input stage of the RF circuitry in the headunit actually forms part of a tuned circuit with the aerial. But I don't think many hu's actually have a tuning tweakable built in these days. Are you sure the connection of the aerial lead into the back of the hu is good, aziraphale? Seems a bit of a coincidence that the reception got a lot worse at the same time you fitted the new hu. Could be something as simple as something metallic touching the RF connector at the back of the hu, when you pushed it all back into place after connecting it up.
first time i fitted the new headunit, i pulled it out again and checked it, because there is a lot of wiring, and it was all "stuffed" behind the unit in order to get it in.
i tried swapping the units over again, aerial reception was fine with old unit on a "round-the-block" drive, but with the new one, it loses signal as i get onto the road behind my house, and anywhere in york it is off and on.
i thought it was the amplifier unit behind the C pillar, as it was mentioned due to me asking about the demister interference aaaages ago. the new head unit is the same brand as the old, but newer and far superior.
2 things - although the new unit is superior in some ways, is it actually superior in terms of the RF performance? Maybe the radio is not as sensitive as the old one?
Secondly, did you try the reception on different stations - was the radio performance always better on the old radio? The reason I say this is that if you were listening to say, BBC R2, there are generally 2 or 3 adjacent frequencies you can pick it up on, you may have had the new radio tuned to one from a more distant transmitter.