Yes, use the correct 10 ohm speaker if you don't want to damage your original radio.
The fact that there is a speaker grille in the back seat cushion of your Parisienne doesn't necessarily mean it came with a rear speaker, it means that it was roughed-in for one in case it was ordered new or the dealer up-sold a buyer one later.
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67 Chevelle Malibu Sport Coupe, Oshawa-built 250 PG never disturbed.
In garage, 296 cid inline six & TH350...
Cam, Toronto.
I don't judge a man by how far he's fallen, but by how far back he bounces - Patton