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I found that a factory 4x6 speaker out of a Sunfire (or anything else that has one that size) bolted up to the OE speaker plate in my 1976 F350. I only needed to drill 2 holes for screws. $5 and sounds good for a factory AM radio!
Heres where I order a lot of my stuff. Make an adapter plate to fit whatever size speaker youd want. Otherwise just get rid of it and only use the doors, I stuck tweeters in that location under the dash.
Why are you replacing it? If you have the stock AM radio, then I see why. In this case, I installed a single 4" co-ax on a black plastic plate with two holes drilled into it. If you have more than this, it will be just as good and better sounding to install the speakers in the doors.
I know CB radios are passe' for the most part, but back when they were still useful I wired that speaker to work with the CB, since I had speakers in the doors. Now it just sits in there and does nothing.