This plastic trim piece that covers the defroster vent is loose on the right side of the dash. Is that a speaker under there? What is the little thing to the right of the speaker?
I have a Vanden Plas. Basic sound system. My door speakers need to be replaced. Is this speaker worth replacing? On the back deck there are tiny speaker grills on the outer edge, 1 per side.
Are they worth replacing?

 

Yes that's a tweeter. The plastic bit I think is a snap-in peg (which is probably why the trim is loose). The tweeters are removable with a quarter turn twist.

In your base sound system, all four of the tweeters in the corners are in use (they are different than the premium system tweeters). In fact, the tweeters of your base system are better and more durable than those of the premium system. If they are all working (which they probably are), I'd keep them as they are. It is the door speakers that fail and don't sound so good anymore. Your system doesn't have a subwoofer either, so try for 6 1/2" door speakers with the lowest frequency response you can find.

The tweeters in the premium system in my car were ALL dead, and I replaced them with the corner tweeters from a non-premium system (for an exact fit). They sound really good.