I've read a lot of guys saying that their VSS isn't hooked up or working. so my question is ... what is it? how do I tell if its hooked up and working? what does it do?

thanks

Your manual will tell you exactly what it's purpose is for.

But in general, VSS allows the unit to utilize your cars speed as an additional variable to use to calculate position. VSS plus the built in GPS ensures more positioning accuracy..meaning more accuracy of your Navigation unit. If I recall from reading the manual, VSS is especially helpful in aiding positional accuracy when driving up/down elevations, since GPS can't really calculate elevations all that well.

So VSS is helpful in getting more accuracy out of your unit.

so how do I know if mine is hooked up? I didn't install it but I should have. I paid someone but had to redo or do stuff that either wasn't done or done wrong.

does your vehicle dynamics work? it should show acceleration and speed and voltage, if not it should be in a setup menu with the status of the sats. and vehicle speed, which beeps with vehicle speed. should be in the manual of how to get to it.