Just n case it wasn't obvious, it's the RealFeelPlugin.ini file in your main AMS folder. You can change the MaxForceAtSteeringRack to a negative number (or positive if it is already negative) rather than adjusting the main slider in the game (since it affects all cars). One you run a mod car, a default entry will be produced for it (often with the backwards FFB). The numbers above are the preferred settings as offered by the author of the mod, but you can fiddle to your own liking. I use 4500, for instance.
The RealFeelPlugin.ini needs to be manually backed up if you use mod cars because it gets reset with most Reiza updates to only include the cars that come with AMS.