This is a quick test I did to see about replacing vehicles in the game with other vehicles from other games. This one replaces jimmy gibbs' car with the delorean from back to the future. Eventually it will be a complete replacement, rig...
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Installing L4D2 Add-ons for Windows
Step 1: Download the add-on. All downloads are in .zip format. (You need to open the .zip archive to access the .vpk file.) If you can not open or extract the .zip/.rar file, download and install winrar for free.
Step 2: Install the add-on. Double click the .vpk file to automatically install the add-on. Or, Right Click > Open With > Browse > \Steam\steamapps\common\left 4 dead 2\bin\addoninstaller Manual Install Copy the .vpk to your L4D add-on directory. If this folder does not exist, create this folder. \Steam\steamapps\common\left 4 dead 2\left4dead2\addons\
Step 3: Enable the add-on. Once you have successfully installed your maps you can enable or disable them at any time. 1. Open L4D / L4D2. 2. Click on the 'Add-ons' option to load all installed addons. 3. From here your list of addons will be displayed. Check / uncheck addons to enable / disable. You can remove add-ons by deleting them from your /addons/ folder.
Installing L4D2 Add-ons for Mac
Some add-on's might have file-names too long for the Mac to recognize. In this situation try renaming the add-on to a shorter file-name and try again. Manual Install Copy the .vpk to your L4D add-on directory. If this folder does not exist, create this folder. \left4dead2\addons\
Installing L4D2 Add-ons for Linux
Manual Install Copy the .vpk to your L4D2 add-on directory. If the folder does not exist, create this folder. /home/username/.steam/steam/steamapps/common/Left 4 Dead 2/left4dead2/addons (Replace username to the name that you log in to your Linux distro)