The familar problem, with some new details:
The front lower door speakers were replaced a year or two ago with two nice NOS ones and they made a great difference. Two decades of hot, cold, and humidity aren't very good compared to dry, cool-stored originals. If I wanted aftermarket ones, that wouldn't be a big problem either since the front takes 4-ohm rounds which are easy to find.
Now, the rear--that's different. Mine work, but they sound muddy as you'd expect after all that time. Nine years ago when I got the car they sounded much better.
Shop manual is half-right, half-wrong. The speakers are 7-inch rounds with a special oval plate and mounted from below, not 8" rounds mounted from above as in the shop manual. Good thing, too, or it'd be necessary to tear out the shelf to get to them. This way, two screws and they're out.
Radio diagnosis section says 4-15 ohms is acceptable for nominal impedance measured with an ohmmeter at the rear speaker.
Speaker p/n Delco 16149126 is stamped 9.5 ohms. That p/n is not unavailable anywhere except from bcacoustic.sk, which looks like a scam--Google Maps shows their address is a house in the middle of Slovakia.
The speaker likely is the same as many others, with its own p/n because of the particular ovalish mounting plate, which is designed for four screws but uses only the two rears since the front of the speaker slides into place. It wouldn't be hard to find or fab an adapter for a 7' or 8' round, or even a 6x9. The plate on the OEM speaker is about 7x9.5 inches, and the mounting holes are in a 6.5 by 4+5/8 rectangle.
So, an acceptable replacement is 10 or 8 ohms, but the market says fuggedaboudit. Every relevant size is 4 ohms or less. I guess they're trying to sell high-powered replacement heads/amps to the vibrating trunk crowd. I did see some 8 or 10-ohm 6x9 speakers listed on Amazon or Ebay, but the listing seems sketchy.
Feelin' like a dinosaur. I don't want ****ty-sounding music or a butchered system. No painfully bright highs, stereo-wars bass, overheated amp, half-assed spliced-in resistors, or something new in the dash (I like the stock look and it works fine). I don't want to change anything except to get decent new 8 or 10 ohm speakers that will let me understand the news voices on NPR and roll down the highway with Pink Floyd's Meddle cranked. My home stereo goes to eleven, but that's not necessary in the car.