I bought a rear bumper cover off a 99 C230. It has a module in each side of it with a cable THAT WENT into the trunk.
What are those modules for ?

You're in the wrong forum - you should be here:

https://mbworld.org/forums/c-class-w202-27/

But, my guess is they are antennas for the TeleAid system. If your car does not have TeleAid, then don;t worry about it. if it does, you will need to connect them or else you will get TeleAid errors. Even though it uses the old analog cellular system that was shut down over a decade ago, it still runs test and will throw errors if it can verify that the antennas are present.