Louie,
You have reached the point of dilemma of our favorite w113 pagoda benz'. The audio systems in 1971 in mercedes, ferraris, and porsches kind of sucked. So to be original and correct, you have to tolerate that. The stereo units used two 4" speakers in the front kick panels in front of the door. By 1970, the metal car frame where the speakers go had circles and screw holes cut in. Prior to that, mounting was a ****. The holes can only acommodate smallish speakers, so today's larger magnets won't fit because of the depth of the fender.
With 7watts per channel, the amps in these early stereos were weak, but matched well with the early speakers. Today, only Becker audio in New Jersey sells the only ones that work well. Otherwise, the newer speakers which are made for 50w per channel only work if the volume on the stereo is turned up all the way, which results in distortion. The good news is that with the old amp and 4" speakers the audio is fine, unless driving with the top down at highway speeds. It's almost good when the hardtop is on. And you can buy an ipod 3.5mm heqdphone type jack input that plus into the back of the radio (through a DIN7 jack that was designed for rare 8 track tape decks) which is actually great.
I wrestled with this, and I chose originality. I love music, and do regret not having full sound in the car at all speeds, but i wouldn't do anything different at this point. I will try a bluetooth wireless speaker and put on my rear package shelf when I drive and see how that works. I'm told they are great.
Since you already have holes and speakers, you have options.
1. If you wanted to be totally pedigree and original, you could repair the holes in your trunk. Whoever did them made a mess of them, so you'd have to have an experienced metal worker weld sheet metal back in there. Then get a Europa II unit (pinstripe) with amp and ipod adapter and 4"speakers and fix the kick panels and rear carpet.
2. Or, leave the rear and front speakers, get a correct Europa II (pinstripe) with the amp and run power and wires back to the trunk amp you had/have to feed the speakers. Many respected members have this setup, (but I'll bet they don't have 9,000 miles on their cars). If you ever get to the point of showing the car, you can always do the restoration work when you are older and have more cash flow.
The website can teach you more about your options. Also try:
http://www.beckerautosound.com/Speaker/BMW-Classic%20Euro%20speakers.htmlI was dead serious about the master technician route to get the car in shape. The people who worked on this car 40 years ago are most likely dead (apologies to those still alive). Dealerships won't touch these things either, and often use "available" parts, instead of correct ones.
Best of luck, and make us proud. You do have a potential museum quality example of our passion.
Mike