This mod replaces the mech. It is only for single player, unfortunately.
How to Install
First back up the ClientArchives.con and ServerArchives.con.
1. open the ED209_for_BF2142.zip and drag and drop the files into ..\Electronic Arts\Battlefield 2142\mods\bf2142\ . which will overwrite your ClientArchives.con and ServerArchives.con files and add a ED209 folder.
2. select the map you want and open the server.zip of the map and navigate to(example) server.zip\GameModes\gpm_sp\16 and extract the GamePlayObjects.con file and uncheck the read only and put it back into the server.zip. backup any maps first before modding them.
3. open that GamePlayObjects.con again and change any vehicle name to ed_209 .example>
ObjectTemplate.setObjectTemplate 2 US_HEAVY_MECH
ObjectTemplate.setObjectTemplate 2 ed_209
Thats all, it should now be in game.