Update: It's released!
I've recently started working on another unmarked LSPD Caprice, this time a newer version with a Federal Signal Spectralux ILS and a CN Signalmaster. It's based on the light setup on LAPD's newer unmarked cars. It appears that the LAPD actually (rarely) uses unmarked Caprices: https://www.flickr.com/photos/146280704@N07/31996010736/
It will probably take a while until it will be released. While the lighting equipment should is mostly final, the car will be outfitted with updated interior equipment which will take some time to be completed. I currently plan to add a console only by default and add a partition as an extra part that can spawn randomly or by using a trainer. I also have another extra part left, if anybody has ideas, feel free to post them.