No power from new batteries

Thanks, I think I found the problem... It was the installer (namely me). When we disconnected the old batteries, we failed to take note which cables were connected to what terminals on the batteries. There was one cable left disconnected (and somehow was hidden from sight). By the time I bought new batteries and installed them, we forgot which cables go where. Now my question is: There are two Red wires and one Black wire. Do I assume that the Red are hot (Pos) and the black is (Neg)? Also, there are only two 12V batteries - does it matter which battery the cables go to?
Thanks again,
Matt-NY