Similar to Cptn Ricks comment - you should be able to flip the 4 pin vehicle connector around and plug it into the 5 pin trailer connector with just the last pin connected. Ie, plug the end pin of the 4 wire plug to the end pin of the 5 wire plug. If you then turn on your lights, this will allow backing up. Note - none of your other lights (signals, breaks, running) etc will work when connected this way - so only do it for backing up. As mentioned earlier, the ground will be through the trailer and hitch.