The USB adapter should identify a positive and negative lead or terminal. In your case with positive ground, tie the positive lead to a ground point and the negative lead to a hot line. In the series V, and also Series IV the switched hot wire is green. Your harness should have the same convention. You should put a fuse in this line.
There are several events coming up where you can meet other owners and get some hands on advice. The United will be in Ontario if you can wait that long. We may make it to the show in June at Clayton NY, depending on which car is in the best shape for the trip.