How do I build an amplified speaker

I have a Garmin sat nav in my car, it has a built in speaker that I can't hear even on full volume. I want to build a single amplified speaker that will plug into the 'audio out' socket on the satnav. I will have to power it from a new 12 volt supply. I only need about 10 watts.
How do I go about building a unit like this. I can follow instructions ok....I just don't know what parts to buy.

Thanks Brian.