I honestly don't think that you can. You could try replacing the center speaker in the dask, but that's a real hassle.

She really isn't too loud to begin with.

 

Pull out the center speaker in the dash and place a small 2-channel amp there. Bridge the amp and replace that 4 inch speaker with a slightly better quality speaker. As for the amp, I'm thinking you can find a cheap 20-40watt max amp at a no-name car stereo place for next to nothing.

Run the speaker wires into the "powered" inputs of the amp and then out to the center speaker. I'll also boost your radio signal to that speaker though, unless you have an aftermarket stereo with fresh speaker wires going to other speakers.