I pulled my back seat out and was surprised to see a 5” round speaker centered in the grille on a factory-looking fitted pressboard piece. The speaker grille obviously has mounting holes and space for a 6x9, which I installed. But the mounting holes showed zero signs of ever having a 6x9 speaker installed, ever. Was a rear speaker an option? There was no radio when I got the car, so I’m not sure what it was delivered with. Thoughts?

The grille shows no signs of having a speaker installed. The one I removed was mounted on pressboard that utilized the edge attaching pins and speed nuts.



The factory size 6x9 bolts right into place. I found no evidence of a speaker wire coming from the dash to the rear.

 

6x9 was pretty much standard GM rear speaker size back then and rear speaker was extra cost option for AM or AM-FM mono radios, even WonderBars. My guess is someone salvaged that 5" speaker from a house radio and made the pressboard mounting plate.

I have seen 6" round speakers in some early 60s GM cars with rear speaker mounted in the rear seat backrest.

See if there's a dark blue wire running from rear speaker area to the right front kick panel of the car, and then underneath the dash to the radio area. There's no separate ground wire as the rear speaker grounded to the car body thru its frame. If it's not there the car probably came with only a front speaker.

 

That's exactly what I went digging for, and I found no evidence of a blue wire running forward under the passenger seat. How crummy would it be to get a car with a rear speaker cut out and no rear speaker?? And you're probably right about the speaker that I found being someone's home radio leftover...