In a module-core program, how should modules interact with each other?

Question

Short background: I'm using MEF/C# for developing a program that has one core (singleton) and multiple MEF parts (I call them modules, each module is a separate assembly with one class that contains the MEF export). The core imports all parts that are needed/available and initializes them (makes them read their configuration files, that stuff happens in the constructors, business as usual)

Now, my problem is that I need modules to interact with each other (say, module A, a graphical user interface, needs to get some data from a database, it'd need to somehow call a function on the database module (module B).)

How'd I do that? Sure, I could just make those parts/modules public in my core, add a reference to the core in my module and be done with it, but somehow, that'd feel ugly/bad. (mainly because I've security concerns over such an implementation, everyone could just call that function, unless I use something like this, but still it doesn't feel right).

Another approach would be to put all of those functions into the core, where they just call the corresponding function of the module. That feels bad, too because I'd have to add a lot of stuff to my core and in some cases that'd be something that a potential customer won't ever use because he doesn't need/have the specific module.

Does anyone know a better method of doing this? Maybe my train of thought went completely wrong somewhere?