I am relatively new to LSPDFR but it would be cool if the classic police vehicles could have interior emergency lights so that they can be used as slick tops or converted into unmarked vehicles to roleplay as federal agencies and such.

I am not sure if this is possible however it would be pretty cool of it was possible.

You can either find addon/replace vehicles that are slicktop and designed after the original ones. Or you can use the existing police4 which is an unmarked slicktop. It can be found in the LSPD section in the garage I believe. And if not there, the FIB/IAA section. Or it can be spawned with a trainer.