1. Download Bag Decompress[github.com] and extract it anywhere;
2. Go to your saves folder at C:\Steam\userdata\xxxxxxxx\228380\local\wreckfest (or whatever drive Steam is installed on);
3. Take cars5.ccrs and drag it over BagDecompress.exe;
4. The tool will create cars5.raw.ccrs - Open it with HxD or any Hex Editor of your choice;
5. Follow the image guide (click to enlarge):

Use the Decimal to hex conversion table[www.rapidtables.com] if you are not sure what value you need to use for the number of vehicles in your garage.

6. Once done, rename your file back to cars5.ccrs and put it back in the saves folder;
7. Launch the game. If done correctly, game won't crash, will automatically compress the edited save file and you will have your extra vehicles in your garage.

STRmods for showing me how to do this.