96 explorer eb adding 4 pole trailer

I am adding a 4 pin trailer connector. My problem is the brake lamp and turn signals are on seperate circuts. So when I connect these circuts together in the 4 wire harness it back feeds through the brake lamp if turning or the turn lamp when braking.  So I tried to add a diod to only allow power to flow to the harness for the trailer but still not go. There are companys that make a conver box  to wire in but I know it is just a series of diods that wont allow power to "back feed"  so what am I missing here?

How many diodes did you use?  If more than 1  how and where did you install them?