im almost positive kent is completely right. you have the VSS wire patched into the tach wire.
in all honesty, i have found that my N3 is incredibly accurate with speed without my VSS attatched.
and i wouldn't let best buy install an air freshener in my car. i understand there are those good installers at best buys, but overall they're craftmenship with car audio installs is f-ing horrible. check out [url=http://www.bestbuysucks.com]www.bestbuysucks.com[/url] , its rediculous...
anywho, hope this helps...
...skeeter...