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Gm did in my Yukon , rear uppers and back doors are on same channels and whole front door is on the fronts. My 4x10s were 9.5 ohm and so we're rear door speakers . No external amp was in mine either, all off of factory tape deck with slave cd . I eliminated the 4x10 and just upgraded doors with type e alpines.
1998 Yukon sle, I pulled origiona stuff myself, have been installing for 25+ years, I've had to integrate into factory amp many times but isn't one in mine. Just a basic 4 wire output like every other gm, same color codes even. Wired rear speakers normally and the 4x10s above rear barn doors and rear doors came on, so I just clipped wires to the 4x10s ...wish it had an amp I would have built a xover and used it for tweeters. And no Bose just tape deck with slave cd, no rear radio or anything like that.
Measure and find out. "Rated impedance" is generally a bit more than DC resistance, as measured by an ohmmeter. If they measure 9+ ohms, they're probably "10 ohm" speakers
I asked the same sort of question when I had a speaker fail on my '93 Lumina. I was told that the standard GM radio/cassette unit was compatible with 4-ohm rated speakers. I slapped in a pair of aftermarket 4-ohm speakers and got better sound than the cheap OEM units.
No way to know. "Louder" or "quieter" is not dependent on the impedance. It's dependent on the sensitivity/efficiency of the speaker. IF (big IF) your electronics are not compatible with the reduced impedance, there could be distortion.
