I sent a request into the following address:
New York City Police Department
Personnel Resource Unit
1 Police Plaza Room 1014A
New York, NY 10018
I suggest you include as much information as you can - all I had at the time was his name, a possible death year and his addresses from the 1900 and 1910 censuses.
I also sent in a self-addressed, stamped envelope - not sure if they prefer it or not. They did not use mine and sent the info back in a large envelope.
I'm very happy with just the cards I got back as it tied a few things together for me. But, one of my cousins received a package back from the NYPD with his great-grandfather's records and they were a few inches thick and had a photo - so you never know what you'll get!
By the way - don't forget to search for your GGF in the NY Times historical papers as well as try looking him up in Google Books.
The municipal services published roster and assignment changes in the Times, so you may get some extra hints at his precincts or time taken off in those.
On Google Books I found some old almanac listings with my GGF's name as the secretary of the Traffic Police Benevolent Association. It turned out they still exist and a current board member was actually able to send me a photo of their first muster. Neat stuff.
Best of luck!