Fitting rear speakers in a 156 is not the easiest task I've ever attempted but the improved sound is worth the effort.
The genuine speaker covers are available as a part from AR. You may have to help the parts people in identifying it in the parts manual. Initially the parts dept told me I would need a whole new rear shelf but the car salesman I bought the car from knew the part number for the covers and ordered them for me. Not cheap though. All the other parts you'll need to attach the covers plus the mounting brackets for the rear shelf should also be available as parts but I did the hard way (DIY).
To install the speakers I removed the rear seat so I could get the real shelf out. After cutting the blanks out of the shelf I secured the covers with thin wire. The metal shelf behind the seat is not a solid sheet - it's only a frame with various holes for mounting thing like the centre seat belt etc. I made mounting panels for the speakers out of 12mm plywood and bolted them in place. Wiring wasn't a problem as there are plug for the speakers already in place. I simply plugged in my speakers and viola I had sound.
The whole job took me a week of spending a couple of hours every night after work so during that time the car was unusable (two car family = no problem). I'm very happy with the end result. Better than having the local car stereo shop do the job (rough as guts). But if you can't afford to have your car off the road Alfa could probably fit them for you using the "real" parts in less than a day. However, the total cost to me was covers, speakers, time and some skinned knuckles.