If you have the Premium Sound System then yes you do have an amp. You will also have a sub, they are in a unit that mounts onto the underside of the rear shelf between the rear 6x9s, you should see it next to your spare tire. I don't know if you can or how easy it would be to use it in an aftermarket system.
maybe this can help, Ford Crown Victoria Stereo & Radio Installation Tidbits
if you want to be 100% sure, you could ask someone at a shop that dose radio installs, they should be able to help.