Audio Systems: Outside Speaker

Question

Outside Speaker   Hi, I'm a wrecker driver in Oklahoma, with the weather I hate taking my phone out of the truck. I wanted some advice on how to wire a speaker. I have a 100 watt siren speaker and want to connect that to my phones 3.5mm jack. My phone rings out that connection I also need it to connect to a small speaker inside the cab. I know I'm going to need some kind of amp, but I don't know what that will work with 12v system on the truck. I am pretty knowledgeable about wiring and reading wiring diagrams. Iv worked for GM for about 2 years.
Thank you!

Answer
Why the outside speaker ? to play music
because you can answer the phone and there is no microfone outside

What about a bluetooth kit and you can keep the phone in your pocket

http://www.oemautosound.com/p-469-bluetooth-media-integration-kit.aspx

or Bluetooth headphones?