if they are stock than you will probably blow them if you hook them up to an external amp. stock speakers are not designed to take a lot of watts even 40 watts would probably damage them or blow them. You'd be better off replacing the speakers and running them off deck power they would be louder then the stock speakers now
remove your speakers and look on the bottom of the motor structure to see what the nominal power rating is. Some stock speakers, especially with newer cars , come with after market speakers that have a better build quality than your traditional stock speaker, thus have the capability to withstand more power.
Also, to get signal from the HU to the amp, you'll need to run some wires back there as well. Some amps will let you plug normal speaker wire in (called "high level inputs" or "speaker level inputs"). If your amp doesn't have this (and most don't), then you'll need something to convert from the speaker wire coming off your HU to an RCA cable that can plug into the amp.
The NE-774V from David Navone Engineering is one example of a LOC that will do that job.
But basically, you're going to cut the speaker wires just behind the HU, connect up the LOC to those wires... run an RCA cable from the LOC back to your amplifier to give it an input. Then run the outputs from the amplifier (speaker wire) back to spot where you cut the original speaker wires... connect them up, and away you go.