I concur with the above post, do not depend on the ball as a ground. What happens is somehow the other lights find a ground off in the other direction and cause a short. I don't understand this either but I think that is what happens. Obviously make sure no hot wires have frayed and are touching the frame.
I have lost two 'cruise controls' which worked off the brake light, until I found some resolution. Again caution if the lights do not work right, do not push it and think it is going to fix itself, or you will learn the way I did. To further protect against the reverse polarity, I added a diode on each curcuit. Also, one of the two the filaments in the bulbs can break off, touch and cause reverse polarity.