The Motorola and GE speaker suggestions are good if you can find them. If you can't, another good inexpensive alternative is the 20 watt Bearcat external speaker for $12.00 from Walmart with an online order. You can run it to full volume without distortion, if you can stand to stay in the vehicle. Remember, 1 watt is normal listening volume and with the windows up it is probably too much. Why do I say that? Because speaker placement is crucial to intelligibility. Also, the closer the speaker is to your head the less volume you need, which is easier on your ears and prevents hearing damage. Mount the speaker as close to you as possible in a safe location, taking collisions into account.
I also found some good small rectangular black 20 watt speakers with mounting brackets and 1/8" plugs made by Idec called "Micro Smart" on eBay. I have been using them on many of my radios and although they are made for stereo use they work great for voice and allow you to get them very close to your head in a vehicle. I have bought them as low as $1.99 each in groups of 2 to 8 speakers. Buy a bunch for the price. You will be glad you did!
Since space is limited in the the lady ham's mustang, we mounted an Idec speaker on her passenger side visor with two black flex ties close to her right ear. The sound is so dynamic and life-like in that location, it startles her every time a ham on the local 2 meter repeater gives a call. The small size of the speaker is unobtrusive and looks very professionally installed that way.
I am glad most of the speaker suggestions are of the good and cheap variety. They are my favorite!
By the way, police car engines don't rev because they all have automatic transmissions, so even at high speed they are relatively quiet. Unfortunately, the 250 watt sirens are loud
and they have to turn up the volume during an emergency response or a chase. Just the voice of experience speaking.