Copied text from Oovee
To install the mod unzip the contents of the archive into the spintires directory.
Does not disable MP and achievements but people without the mod will not see the trailers so take care.
Other bugs may include not unlocking the garage unless you are the server and winching will disable winching on that truck (until you rescue or visit a garage, not sure about this one)
Oh and because of the odd trailer attachment point you will not be able to reattach it unless you leave is on a hill or cliff…