Veh mod threads:
http://forums.tripwireinteractive.com/showthread.php?t=36237
http://forums.tripwireinteractive.com/showthread.php?t=46417

Vehicle mod source code:
http://www.moddb.com/downloads/mirror/27271/112/20b437d3eb9c1b5e3fee9933f5236a8d
(or see the threads above)

Crashes:
http://forums.tripwireinteractive.com/showthread.php?p=1291055#post1291055

ServerPerksMut fixes a crash when someone leaves the server while being in a vehicle. Just as an info you might need to know that.