This is my BMW MX5 Unmarked. You can change the plates easy with editing the texture. 

Install location:
GTAV/Mods/Update/X64/DLCPacks/Patchday9ng/dlc.rpf/x64/levels/gta5/vehicles.rpf

Credits: 
Police Lighting installed by: JustRPFunMods Iddo 
Plates By: JustRPFunMods Iddo
Model: Turn 10 Studios (Forza Motorsport 4) forzamotorsport.net 
Covert: Denus