PA speaker

I am considering installing a PA speaker with a Cobra CB radio in my Nissan truck... has anybody done this before? I'm wondering how loud it will be, because it will have to go in my engine bay so it can stay dry. There's not a lot of room up by the grill, so it may get jammed near the firewall, but will it even be loud enough to hear over the engine noise?

Also, does wattage of the speaker/CB radio matter?

Thanks

Okay so it's not super loud in the engine bay, it's about 2/3 of the way back, closer to the firewall. Would mounting it actually under the car somewhere pointing to the ground be louder? It would get water on it then though...