The stereo I posted would probably run 80-100w, and that might be enough for an M.
It comes in the later model FWD New Yorker 5th Avenue and Imperial.
Maybe it really is so simple as a nice single DIN with Bluetooth.
*edit
My 1989 M-Body 5th has 6x8 rears, and 3.5 fronts? Anything special about them?
It's not a power issue. If the interior speakers don't have proper enclosures on the backsides you'll never get any low or mid range sound out of them. I only brought up a sub in the trunk because it will round out the sound and make it sound good. Properly setup interiors don't need subs, but the factory setup on these cars don't make it possible without a lot of work.
The dash speakers are a chore because heater ducting gets in the way. They are 3.5", but aftermarket speakers have larger magnets. Getting those to fit will require some trimming. I have some old 3.5" Rockford Fosgates in mine. Still though, no enclosure underneath. I've seen people install 6" round speakers in the kick panels on the door, but I've never done it.
The factory rear speakers on Fifth Avenues are 5x7 although 6x9 do fit. There is less room on the deck as opposed to Diplomats and Gran Furys because of the landau top. My Dad was able to mount 6x9's going through the top on his Diplomat. On mine I had to mount them from the bottom and they're covered with cardboard and fabric as a result.
I was able to squeeze enclosures on the backside of the 6x9
pair I have are actually out of a late 90's Chevy car. You can see the bottoms from the photos. It's tight though.
If you wanted a sub setup like what I have it involves cutting out the spare tire mounting hardware.
I hope this helps.
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