led fit to trailer problems

Replacing all normal bulb lights on my trailer with LED units made for the job - in 12V of course. All connected correctly as per normal bulb style the tail side of stop/tail unit will not light even though it has power to its lead, good earth, and works on its promotional battery in sale package off trailer. Tail will only will only work if I disconnect the pos side of the stop circuit. The stop/tail unit has three wires as in earth, stop & tail.The indicator is separate unit and works well sharing the earth wire for all units, 2 or 3. I have since added a third LED unit of same part number for test purposes so that one stop/tail does the stop light, the additional identical unit does the tail function. In this config all lights work as expected. http://www.autotecnica.com.au/lighting.html lights appear under the unit labeled JH1700S units concerned have no part number on this site. I understand electrical until electronics comes into play then understand very basic but willing to learn. Must be electronc approach to resolve this issue I have. Thankyou thankyoumy other car is a SUBARU too!

Without analysing fully to resolve the location, it sounds like an ground issue - ie, and interchange of ground (centre) and one of the conductors. It's easier to pick with normal bulbs because the faulty connection will cause the other bulbs to light up (dimly etc).

I found the answer with help of a diode. Interchange of ground is a good technical description. If this unit had an earth wire and separated from each other internally this would not have ocurred.Problem due to different switching within the car as worked without car input. I added 1n4007 to pos input to brake light on each side. Now works as expected. Thanksmy other car is a SUBARU too!