Hi Stripes,
Today I just installed an AUX to my 59 plate GP fitted with B&M.
First off you need to check which model your radio is. If it's Blaupunkt, you need a cable with a blue ISO connector, if your radio isn't Blaupunkt then you need a cable with a yellow ISO connector.
You can get them on Amazon although they're cheaper on Ebay- just search 'Fiat punto AUX cord'. My one was about £4 if I remember correctly.
Installing the cable:
Pull your radio out using the keys (locating holes are above and below the volume knob and the arrow pad)
You should then see the leads for the power & speakers and the USB port on the left with the fuse and on the right you should see the aerial lead
Take a look at the USB port leads, they should be connected to the side you want to put the AUX connector in
Remove the ISO connector for the USB leads (take care, you need to push a little tab in to pull it out)
Plug the ISO connector end of the AUX cord into the slot you've just emptied
You can then route the cable around into the glovebox (I found it easier to push the ISO connector into the radio compartment from the glovebox)
Making sure everything is properly plugged in, firmly push the radio back into the head unit and take the keys out
Turn the ignition to the first notch, plug the AUX cord into your phone and select 'Media Player' from the B&M menu on the dash
Select 'Folders' and then press play on your phone's music app
Your car should now play your phone's music!
It works with third party apps such as Spotify and youtube as well.