My husband (back before he was neat) and MIL trashed two cars (they switched the titles on the cars between each other multiple times, and he had to look after her car for months at a time when she was in Florida or sick/disabled, so it was caused by both of them). They weren't packed with trash to the ceiling but both of them were absolutely covered in disgusting mystery stains, to the point where it looked like a murder had taken place. There were literally coffee splatters covering the ceiling. There were crumbs under the seats (not only crumbs but for example I once found an entire salad's worth of dessicated years-old crispy noodles scattered under the seats), trash everywhere, completely random items that had nothing to do with the car ending up in there, etc. I once saw MIL leave a cup of coffee with milk in it in the cupholder of her car that she left with us and it stayed there for two weeks until I made my husband get rid of it. It was nasty. I begged my husband for 7 years to do something about it, and sometimes progress was made, but it never got to the point of not feeling like you were driving around in a mobile dumpster.
Thankfully one of the cars broke and was scrapped, and the other one needs $5K in repairs, so last month we got a Prius. It's so nice and refreshing to be able to go into a clean car! My husband has actually become somewhat fanatical about keeping the car clean -- he takes it to the car wash every couple of weeks, cleans any dirt from the inside weekly, promptly removes any trash in there, and doesn't leave random items like papers or electrical cords in there anymore. There is hardly anything in the trunk so we can actually transport laundry and stuff in there. Turns out my husband doesn't need 90% of the crap he was keeping in the trunks of the other cars just in case. MIL is still forcing us to keep the second car "in case she ever drives again" even though she has not driven since 2016, still has balance and mobility issues, and has no plans to practice driving in the foreseeable future. As I said, it needs 5K in repairs, so it would be cheaper just to scrap it and buy a decent used car if the day ever comes when she wants to drive.