It would be amazing if cars still had working radios, headlights, and tail lights and if their trunks opened and closed too. It would allow players to mark cars that they have walked by simply by turning the radios on and off and changing the stations on them too. The cars may not have power anymore but neither do all of the buildings that players enter and radios in those places still play music.

Players could use the radios as a distraction, to lure enemies, and vice versa. Some enemies might be easier to kill without luring them when they're located near a radio since some of them may not have good hearing and/or attention span. It may also be easier to sneak by certain enemies because of their poor hearing and/or attention span too.

The same could apply for headlights and tail lights as well but with light instead of sound. Not only could players encounter enemies in an area that just have good hearing but also rooms where some enemies in them that are blind teaming up with enemies that are deaf. It would make great dialogue options and subtitles too. One enemy may be blind and try to warn the other that you are near by telling him or her or it and vice versa. That would be hilarious to experience.

If trunks opened and closed it would add to the combat system. Cars could be rigged by enemies, such as raiders or independent ones, so that it explodes either when the player and/or companions near it or when the player opens the trunk. Some cars could also hold items too like dumpsters do.