Where to put the files?
Unzip the Download file
You then put the blabla.idf into your C:\codemasters\insane\data\vehicles or C:\codemasters\insane\data\maps
DO-NOT Late-Join Online Games with add-on Vehicles unless you are certain the other players have the add on you are going to use
If you’re installing an add on pack from www.play1nsane.info :
Empty your current vehicle or map folder, and replace the contents with the contents in the ‘current add on pack.zip’ you downloaded from this site, this way, every player has the same files.
If you installed insane in a different location
Then just change the path
Example
D:\insane\data\maps or D:\insane\data\vehicles