I've gone through multiple forums and troubleshooted for a long while and I still haven't figured it out. In the linked carpack, I've installed the Single-Player reflective pack. Realizing its an add-on, I've renamed the lspdvic,lspd14char, and lspd20exp, ytd,ymt and yft files to police,police2,and police3 respectively so they work as replacement. They spawn and work, you can modify their extras in the garage, however, POLICE(lspdvic) and POLICE2(lspd14char) only spawn ( when backup is requested) with extra 5 or no extras at all. Police3 however (lspd20exp) spawns with all extras but 11 and 12. Playing around with the vehicles.meta has been futile for me.
vehicles.meta