in the UK most BMW dealers also offered Pioneer stereo systems as 'dealer fit' options. I think there must have been a deal between BMW UK and Pioneer; they even published brochures listing the various pioneer options, which included having decent size (~7") speakers mounted in the kick panel under the rear seat (my 3/87 build had those). These pioneer fitments were considerably more popular in the UK than the OEM factory audio options.
In the USA a similar situation exists; IIRC US dealers would fit the radio antenna into the wing (front fender). That is 'standard' for those cars, but is virtually unknown elsewhere in this model of car.
You can find out if your car left the factory with a factory audio system or not, if you can find the original build sheet details (often on a piece of paper under the rear seats). Similarly if you can find the original bill of sale, it may contain details of any dealer-fit accessories, including radio etc.
Regarding 'top options', there are two flavours of rear headrest cover pods; standard (which accepts a 6x4 speaker provided the magnet isn't too big) and 'premium' (which has a multi-way speaker mounted in the centre of the pod) . In the UK the premium speaker pods didn't appear commonly prior to the introduction of the Highline models. The highline models had 6x4s in the A-pillar mounts as normal but also had an option to have additional tweeters mounted higher up. Early four-speaker setups had a front to rear fader control that is wired into the dash, with or without an auxiliary amplifier. [NB you cannot use this fader if you have a modern amplifier]
It must be said that apart from the provision for an electric antenna, and four speakers the standard audio provisions in E24 were pretty basic. The factory speaker wires (standard parts of the wiring loom) are very skinny and are even installed incorrectly in many cases. You can fit out a UK car with all kinds of genuine BMW audio equipment that was (say) fitted into 7ers etc and it may be period correct. However I don't recall seeing any UK supplied 6er which has had this kit fitted from new; maybe I just have not looked hard enough though.
Options for having good sounds (as opposed to period correct sounds per se) include
- Mounting better speakers in the rear kick panel (I have 6x8" there in place of the original dealer-fit pioneers)
- fitting a subwoofer ported into the boot
- fitting extra speakers into the rear side trims and/or the doors
- fitting tweeters at the front eg on the dash, in the door tops, or in the mirror mount triangle trims (as per E36)
- fitting an auxiliary amplifier eg under the rear seat
- converting rear pods to accept better drive units, with better sealing at the edges and ported into the boot
- mounting 5-6" round speakers in the A-pillars using special mounts (NB this is a bit trickier on a RHD car because there is a padded bolster next to the accelerator that is likely to foul on such mounts).
You will notice that most of the above options are to do with the speakers and involve cutting the car about. This is because the standard speaker options/enclosures are the things that soonest limit the sound quality that you will get; even the 'premium' speaker pods are a bit crap by modern standards. If you fit better speakers you will soon find the limits of 1980s amplifiers; they mostly didn't have grunt by modern standards, in good part because the speakers wouldn't have been able to reproduce it anyway.
If you are stuck with 4x6" speaker mounts (which you don't want to modify) and want really good sounds then unless you fit a sub as well, prepare for a disappointment; there just never will be much guts to it because the standard enclosures are just not very good. The best 4x6 speakers I know of are JBLs or Infinity ones but these don't fit easily behind standard BMW grilles in the A-pillar, and in some case the magnets are too big to fit in the opening in the metalwork too. Mounted in the right enclosures, these drive units can produce a decent sound, but the enclosures on an E24 fall well short of what is required.
So you need to decide on your priorities and act accordingly. For a long time I have had a cheap modern head unit and four decent speakers, properly wired (including 6x8s at the rear etc) which cost a lot more than the head unit did, and this sounded pretty good; many others have said so. I could add about 100lbs of other gear which would give me a bigger better sound but this is marginal gains stuff unless you are doing it for the 'benefit' of folk who are not actually in the car....