Why don’t my woofers sometimes work but my tweeters do
Because your woofers are on thier way to the “speaker’s grave.” It is time for new speakers for this 10 year old car. A little early but GM speakers have gotten much worse over the last couple of decades.
What size are they-i can get a set of aftermarket-might be better
You are just going to have to remove the ones you have and measure them . Your vehicle is 10 years old so without seeing them they might not even be factory.
Pat, go to Crutchfield.com and look up your vehicle and the speakers listed for direct replacement and any adapters you might need to mount them. Order a new set from them or the same parts from another source. Although Crutchfield will send you everything you need plus instructions.
If you are getting zero sound out of the speakers, it’s more likely to be the electronics.
What @Mustangman said. Even speakers which are the same size as yours might have their mounting holes in different places, which means you need to buy or make an adapter to get them to fit your car.
Sounds like you need new woofers. For the same sound amplitude, the flexible surface of a woofer speaker deflects greater distances than a tweeter. That greater movement will gradually deteriorate the speaker material. I had to replace some speakers in my truck recently, for this same reason. The speaker material was starting to fall apart. Speaker technology is better today than in the past, and I think you’ll find once you install new speakers the sound quality will improve, even compared to when your woofers were new. Mine certainly did.