A few years after I purchased the car, MP3 players were starting to become mainstream, and because this car has "collector" status, I'm not allowed to install a new stereo system. It would have to be a Becker, and look essentially authentic. I did most of this work 12 years ago, but just added bluetooth last summer.
To get sound without installing a real stereo, I put a 2-channel amp under the driver's seat. You probably can't see the detail, but there's a 12V socket assembly on the side, and then a bluetooth receiver that is stuck to the side of the centre console.
The speakers are not permanently mounted, as I used an existing screw hole on the passenger side to affix the right speaker, and then made a bracket for the left one that simply slides overtop of the vinyl panel and tucks snugly out of the way.
I also put a small, powered subwoofer under the parcel deck. Yes, I did drill some holes in the plywood, but of course, the carpet covers it back up.