Uniden and Radio Shack sell external speakers that have built-in amplifiers. They are powered by the vehicle's electrical system. I've used both brands over the years and found them to be quite effective.
Uniden 4" Amplified External Speaker (BC23A)
I don't know whethert you could feed all the audio from several radios into one of these speakers without causing damage. Personally I'm a "one radio - one speaker" guy. What I've done when I have several radios in one vehicle is mount speakers in different locations to help tell which radio I'm hearing. For example, if the sound is on the left, I know it's my scanner and if it's coming from the right, I know it's my amateur radio.