anybody know much about these?

One hole is for 12v positive, one for 12v negative and one for each left and right speaker channels.
Speakers were grounded to the body.
You can open it and probably get a good clue as to which is which, because you don't want to put 12v into the output amp.
Just take those 2 screws out and you should be able to tell which wires are which by color. (red 12v pos, black ground and the other two speaker positives)
You might get the left channel out the right side speaker at first, but it won't hurt anything and you just switch the wires left for right.
It originally had a plug (40 years ago) :D

I take it you know how the 8 track to FM adapter works.
Just plug it in like you would a tape , turn the volume up and tune it to a station.
The only thing you seem to be lacking for your old school beats is the FM signal booster so you can pick up stations a lot farther away. :D