From what I'm reading sounds like you have a vent there but its not a powered vent ????
so what you need to do is get the fantastic fan you want (I would recommend getting one with a remote control ).....and the setting for air in or air out .
make certain it has the rain sensor on it -----get some caulk which is self leveling and make sure you have assortment of screws .
then you should be able to pull off your in- side trim offf the current vent, and that may help you to see what's inside there .
pick a time when it's not going to rain---- pull off your current vent----run wires from a light( there should be a ceiling light somewhere in your bedroom) you should be able to tap into.
Clean up the roof where you pulled the old vent off make sure you get all the old caulk off things like that ----but do not use a real strong solvent on the roof basically a damp rag on a warm day works the best and make sure that if they include a gasket you put the gasket in .
Screw down evenly but not too tight make a good bead of the self leveling caulk around the edges and then up onto the vent plastic itself (the edge )and make sure you put a--dap on the screw holes also.