Currently there is no Steam Workshop for Wreckfest 2, so currently mods are being shared on NexusMods and the Official Wreckfest Discord.

For NexusMods you will need to set up an account to be able to download, but it is a nice place to both share mods and download from as you can see change history easily. I can recommend the Official Discord as you'll be able to see and take part in discussions surrounding the mods there too.

I have now released a Vortex Extension to smooth out the mod install/uninstall process a lot.
Download the Vortex Extension here: Wreckfest 2 - Mod Support at home[www.nexusmods.com] or auto install it directly from within Vortex as it is now officially supported by Vortex. To find it in Vortex search the games list for Wreckfest 2 and click manage to download my extension directly.
Download the Vortex Mod Manager here: Vortex Mod Manager[www.nexusmods.com]


Some mods may have their own instructions for installation but generally each mod is likely going to be an archive (zip/rar/7z etc) with a data folder inside. Likely case is all you will need to do is copy this data folder into your Wreckfest 2 root directory (Where the game exe file is).
Typically something like:
Inside this directory you will see a matching data folder. This is where the data folder from the mod archive will merge into. Some mods will contain some files that exist in the game already so you will need to overwrite the original files to use those mods, more on this later.
After you have copied the data folder and merged it with the data folder in your game directory the mod should be fully installed.

When using mod files you will not be able to join any Vanilla Servers to play multiplayer. Mod files on server and game must match. Only a few visual mods are exemption to that. Not matching will cause the following error to appear when attempting to join: ERROR 300E Server checksum mismatch or specifically: 300E-2060610203.

Note

There is one funny quirk, if you Verify your game files with Steam then add mod files, after you launch the game the mod files will be deleted by DataCleaner.exe which is part of the original game. So if you ever Verify your game files, be sure to first launch the game and close it then install your mod files, this order will prevent the mod files from being deleted.

Another thing to note is mods that replace existing files especially may be broken/uninstalled when the game updates as the game update could replace the mod files. After each update you are probably best to be safe installing each mod again that replaced original files, or if when trying to join a server that is using the mod you have installed and you get either ERROR 300E Server checksum mismatch. or 300E-2060610203 then likely the issue is either your game is missing some mod files or the server has different files.

It's also possible for mods to not be compatible with other mods, especially in the case where two mods overwrite the same files. In most cases the effect from one of the mods will need to be favoured but it could have some unexpected effects if conflicts are ignored.

If you want to remove the mods don't worry, you don't need to uninstall the game and reinstall it.
The easiest way to uninstall your mods/revert back to Vanilla WF2 is to Verify your game files with Steam. You can do this by going to your Steam Library and right clicking WF2 and going to Properties. From the Properties window click Installed Files on the left side then on the right you will see a button labelled Verify integrity of game files, click that to begin.
After the file Verification is complete Steam should be redownloading any files that don't match the original base game files, then once you launch the game DataCleaner.exe should run automatically to clean up anything that shouldn't be there.

An alternate method for cleaning up mod files (and also preserving the mod files to reuse later) is a tool I made which essentially runs as a more advanced DataCleaner.exe. It actually makes use of Bugbear's DataCleaner.exe and any files that are found instead of just deleting them they are moved to a backup folder xModFilesBackup in the WF2 root directory. The tool can be found here on NexusMods: WF2 Mod Files Extractor
Or it can also be found on the Official Wreckfest Discord.
A note for this tool is it of course cannot restore original files if they are overwritten by a mod. For those Verify game files with Steam is the only way to do that aside from your own manual file backups.

Mod authors are free to share their mods where ever they like ofcourse so you could find them in other places than mentioned here. For downloading without signing in to NexusMods I highly recommend the Official Wreckfest Discord as most mods have been posted directly there to download.

As this is still very much early days, there are still a lot of unknowns. While mods can serve as some nice filler content while waiting for official updates, keep in mind mods can cause problems. So if you have mods installed and notice an issue with the game, first try to completely uninstall all mods and try to reproduce the problem before reporting it to the developers as feedback if possible. Problems caused by a mod especially if not mentioned can cause some confusion in bug reports. That said if you find a problem with a mod that isn't known by the mod creator be sure to let them know.
I hope this guide helps anyone in need of more information, it will no doubt be added to and adapted as time goes on.