There may be a problem with the connection to the tow vehicle's brake light switch. When you are not appying the brake, the brake light is not on. The controller sees this, shuts off, and shows zero on the screen.
There is a way to test this. With the trailer hooked up and the electric cable connected, drive along a straight and safe bit of road at a relatively slow speed (maybe 30 mph), and downshift the transmission one or two gears so that you get some engine braking - but DO NOT touch the brake pedal. Is there a change in the voltage shown on the display?
I can think of one other possibilty. Some older foreign vehicles (European, I think) have a weird brake light setup, in which the switch operates in the opposite sense from what is expected in most cars. The installation manual for the controller may mention this, and tell you how to deal with it.
Let us know.
Bill
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