Not sure what Whistler handheld you're using, but for my PSR-800 that was upgraded by Whistler to a WS1080, I used a Valor amplified speaker mic. It has an internal battery that gives you very loud sound without distortion.
Lebrunmn: read your post about the amplified speaker mic from Valor, which model has a single "phono-pug" connection that will work with a Whistler or other scanner? Most every speaker mic I've seen has some type of (usually proprietary) duel plug setup/connection.
You might also look at moving that speaker to another location, like down the door top trim where it would be aimed at the side of your head or even somewhere down by the console are pointing straight up in the cabin.