Get 2 body shakers, an amp, connect them up like regular speakers to the amp and connect the amp to the rear speaker output of your sound card (with regular speakers as front speakers). Use speaker fill on the sound panel to get the game sound also to the body speakers, good e.g. for engine vibes and some sound events. Now also run the provided app to get your seat vibrating with gear shifts and when driving over a curb.
YouTube: See how it works in simulation
Important: This is not production app fitting all environments. I provide the source code. If it doesn't work for you and you have no idea about programming, sorry. Otherwise: Easiest migth be to get fmod package for development (free) and replace the generate_tone source in the example. Also this is my testing app to look at various values and I didn't clean it up.
DriveVibe - Body Shaker 1.4
Body Shaker - Simulate your seat