Re: Bose Stereo
I went through the exact same thing of really naff sound from my Bose; no radio signal to speak of and really poor volume and lots of distortion with the standard Bose setup. Having tried checking the aerial booster, cabling, speakers, setup and generally every connection I could get at I decided it has to be either the Bose amplifier or the headunit. disconnecting the Bose and using the Blaupunkt standard output just made it sound worse, so I thought I'd try a different head but as cheaply as possible. I went for an Alpine ixa-001 with bluetooth unit off the bay on the principle that if it didn't work i could sell it on again.
Difference? Night and day! I wired it up the same way as the Blaupunkt (ie speaker outputs directly to Bose). The result - volume increased, bass to shake my fillings out, and the radio now played a strong signal (easily better than the standard Blaupunkt CD - the aerial worked all along!) and now i get stations I never knew existed! I added a small hard drive (80Gb) and although no CD (but could add a changer if I needed) I now have the ability to work USB/bluetooth stream/radio/iPod.
Being Alpine, the built in standard steering interface controls worked too (no extra connectors etc. to buy). It took a bit of time to "tune" the sound to my taste as there's no manual EQ on the x-001 so you may want to go for a unit that has this facility. Overall though, I just wished i had tried this first as it'd have saved me many hours of grief taking the 156 to bits - as compensation, at least i have a better idea how it's all connected now.
I don't know how any Alpine would work with the display on a GT, presumably this would also require a head unit that had CANbus controls, as far as I'm aware Alpine only do one for OEM Vauxhall/Opel (Alpine Display adapter APF-D102OP), there may be other ICE manufacturers that do something but then you're probably back to needing a steering wheel adapter too.
Bottom line though - standard Blaupunkt head supplied with Bose by Alfa is really poor. Change it, but make sure you get a unit that can handle streaming bluetooth (you might not need it now but you will want it at your next phone change) and you can use your phone as a player as well as an iPod etc.
Hope this helps.