Most vehicles come with a readme file that tells you where to put them. From my time in modding i've learnt where some cars go and here is a list of where most of the police cars go (not sure about FIB as i haven't ever modded those slots before)
Police - x64e/levels/gta5/vehicles.rpf
Police2 - update\x64\dlcpacks\patchday3ng\dlc.rpf\x64\levels\gta5\vehicles.rpf\
Police3 - update/x64/dlcpacks/patchday4ng/dlc.rpf/x64/levels/gta5/vehicles.rpf
Police4 - x64e/levels/gta5/vehicles.rpf
Sheriff - update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/levels/gta5/vehicles.rpf
Sheriff2 - update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/levels/gta5/vehicles.rpf
Policeb - update/x64/dlcpacks/patchday1ng/dlc.rpf/x64/levels/gta5/vehicles.rpf