HU effecting Speed Controlled Volume?

I just installed a new HU in my brother's '99 Camaro with Speed Controlled Volume.  There was an "extra" wire harness that plugged into the back of the stock radio that I assume is for the SCV that the receptacle was not included for on the Scoshe harness he bought to install the HU.  Now the Speedometer is falling past zero at rest.  It is fine when the vehicle is in motion, but the needle falls down at rest.  Does the SCV not being connected effect this.

Thanks for the help.

unplug the radio see if the problem goes away if it does check you wireing hook up only the basics 12V constant,12V ing., and 4 speakers if all is good check the power ant. wire if equiped ,than check dimmer wires amp turn on as thay amy be going to something wrong in the harness. i am not sure i've never use a Scoshe  brand before.