1998 E320 Rear Speakers

[DirtyDan]

Hello, I just recently got a '98 E320 and its I am really loving the car. In good condition when I got it and so far keeping up with maintenance has been all I needed to do since I changed the EGR valve out.

The only thing that has bothered me was that the audio was horrible so I got 2 speakers from a local audio store. They were for the front door and I had to design and 3D print an adapter plate for them to fit but I got them in and I have an aftermarket dash so I Bluetooth my phone to it and it sounds pretty good, was proud I got it done, I never changed out speakers or have done much of anything to cars in general.

Anyways,
Now I'm interested in replacing the rear speakers(Not the rear door but the rear speakers) and so far I've found out that they are 6.5", and that they are subs not regular speakers.

I want to change out the rear speakers but don't really want to deal with an amp or anything(I don't know how to wire them or anything like that and would rather a drop in replacement of some sort)
Does anyone have suggestions on what kind of subs I can put in there, like is there a certain watt I should stay under, or is this something that is even possible?
I don't really want or need it to "bump" but would like a little bass when listening to music, would be happy to have not blown speakers, was avoiding OEM replacements because they are old like the car and probably don't have much life left in them.