I just installed the z2 in my 02 trailblazer. I connected the vss to the green/white wire suggested and the z2 reads my speed fine, the problem is that now my speedo hovers within five to ten mph of what i'm actually doing. I was wondering what could be the cause of that?

Also, when using the metra adapter to retain the onstar functionality does the illumination wire not work correctly? The unit stays in day mode and won't even let me select a night mode.

If these are dumb questions I apologize, this is the first unit i've ever installed.

So you say your Z2 shows the correct speed but the cars speedo is 5 to 10 off. How do you know which is correct? How do you know the car speedo wasn't always wrong. My Z2 GPS speed is 3 mph off of my truck's speedo. so what your experiencing is probably normal, not a result of you connecting the VSS wire. Believe the GPS speed before you believe the truck's speedo.