2Take1Menu allows you to load custom RPF packages into the game without external programs such as OpenIV. Currently, only custom vehicle models are supported.
While the game is closed, place your RPF files into the following directory:
Typically, custom vehicles are loaded locally, meaning other players will not be able to see them (because they don't have the RPF files). However, 2Take1 is able to make these custom vehicles visible to those who also have the RPFs. You can toggle this option in Local Tab → DLCs.