Basically, the same way the elegy and jester have paintjobs in game, you can allow the fire department and police department vehicles to have more than one skin. You have to have the paintjobs in your modloader, custom.img, or gta3.img, but this would allow for private ambulance companies, more than one station marked fire engine, multiple police agencies using the same model, and even fd/sd sharing say a tahoe for command or something.
This works and has been tested on another server which is no longer around, just thought I'd mention the idea.
MTA has it, but for all vehicles basically. I think Kalcor will implement the feature that we can add custom weapons & vehicles, and then this would be possible. But that's for next year.
I'm curious to know how this would work (especially if someone doesn't have the needed mods).
If a player doesn't have the needed mod, they see the normal car. If they only have one paintjob, they'll only see that paintjob. If they have two paintjobs and it's on paintjob3, then it will display the default paintjob0 or paintjob1(not sure which is default.)