i have spent the last two days replacing a new motor on a single step electric kwickee step, the motor works fine using a battery in both directions but when it is wired to the existing wiring it will...
It sounds like the control box is only sending the +/- signal, and not the -/+ signal. When I worked in the trade, that kind of a problem was often diagnosed as a switch not making. In order for the power to reverse, the forward limit switch must make (or something else like a safety switch makes). I would examine the electrical print, and try to determine how the direction switches, then either jumper out, or open the faulty trigger to check the operation.