I use the deputy’s daily log sheet
they add under their notes: vehicle backseat searched nothing located/something located.
as far as snagging the written directive there are a number of agencies that make their policies public facing so you may be able to get one from their websites, or did you try the CALEA sample policy library?
and as far as proofs, I used to have a few officers who were shall we say meticulous about following the policies and putting that language in there so I would reach out to them to ask for some report #s, if no one seems to be doing it as they should be (they may be doing it just not documenting it) I would reach out to all of patrol and patrol sergeants with the exact language as a nice little reminder then request from the sergeants or records that when they read a report that had that language in it to let me know. Worked well for me for 11 years. Hope it helps a little.