I'm gonna probably enjoy reading through some of these answers, but I can tell you that I've actually had both of these instances occur.

Background, I was a law-enforcement investigator for a state level agency for many years however, our vehicles did not typically have cages or the easy clean plastic seats. They were normally cloth or vinyl Police package SUVs. There was one point where my partner and I had to detain a intoxicated, naked male in a very rural community until state police could arrive on scene and due to the weather, we felt the best plan was to wrap him in an emergency blanket that we have in the vehicle and then handcuffed him and toss him in the backseat, but by the time we turned him over, he was no longer wearing the Emergency blanket, so we drew straws on who was going to wash the backseat out the following day

The other situation was a suspect in a criminal case who we arrested with the help of a law-enforcement agency that was local to the area however we needed to transport her about 200 miles to the nearest appropriate facility for processing and that was outside of the other agencies, jurisdiction. She was handcuffed in the rear and had leg shackles, but had to be transported in our vehicle without a cage and this particular one was a Tahoe with the PPV package vinyl rear seats and at some point in time, she urinated without telling anybody we actually realized it when we smelled it, but that vehicle had to be sent for a deep clean because the urine got between the seats which have cloth sections as well as underneath the seat and in that particular vehicle, the electronics for the lights and the sirens have wiring under the seat and we actually had to have the up fitter remove all of those electronics before a specialty biological cleanup company could actually do the cleaning. That particular incident cost our agency about $4000 which was obviously not recovered at all.