Actually..The Ford PI (Performance Improved;
NOT police interceptor) package was for all fleet vehicles.
Ford has not made a "Police Interceitor" in probably 30 years (if ever). This means taxi's were also subject to the treatment. In later years, they used the town car chassis that allowed for something like 4 to 6 inches of rear door space for police cars & taxi's.
Typically the package was heavy duty electrical system to support lights & radio's, late 90's PI's got a different intake (later all versions got the PI intake), a slightly different engine management package, and a stiffer suspension. One of the car mags did a head to head of a stock Crown Vic LS versus a Stock Crown Vic PI (loaded with standard police equipment) and found out the
civi model was 2 tenth's faster do to the extra weight of the police equipment.
If you doubt anything I'm saying, check out
www.crownvic.net for the best info about PI Crown Vic's.
I only know what I know because I've got a 99 Grand Marquis with afew "special" modifications.