Wooden crates

Wooden crates are decor items that can carry around almost any item that fits inside — usually things like food cans, soda cans and car parts, with seemingly no limit. They come in small and large variations.

Functionality

When an item is dropped or placed into a crate, it usually locks into place and will not collide with an already locked-in item, letting it stay in the crate when the crate is moved. Not all items lock into place — some (DVD, side mirrors, small turn signals, sometimes the wrench, and the 6L radiator when laid flat rather than on end) can fly out due to the crate's sudden lurch toward the player when picked up.

When a crate is in or on a vehicle (seat, trunk, cargo bed or rack), placing an item usually locks it onto the object under the crate rather than into the crate itself, and the item may clip through and fall further down (e.g. through a cargo rack's openings). Best practice is to remove the crate from the vehicle, set it on the ground, then place items so they lock into place.

Uncrateable items

Some items seem small enough but will not fit into a crate due to their collision bounding box:
Bar stool
Some tires

Locations

There is always one at home, and more can be found around various points of interest. The bar and gas stations (and other fuel stations) usually have a crate; the California Motel can have 2.