I'm finally replacing the dead AM unit in my truck. Actually going a bit fancy with a radio that has a 3" TFT that is backup camera compatible. Anyway, I plan to mount the speaker in the area below the dash that has a speaker cutout. What size speaker is that? I measure 3.5" opening as best I can tell, but I've read it's either 3" or 4" standard sizes. I'm using that slot, and then I may find a space under the dash for another and fab a bracket to hold it
