Trailer wiring problem

Talking bout a 1994 Ranger 4Cyl.. purchased a T connector wiring harness for 4 wire trailer with included tail light converter. Also purchased the circuit tester that plugs right in to it (with lights to test the tail, left, and right blinkers.)

Heres what happens to the tester

With my truck headlights off, I can step on the brake, and both blinker lights light up (expected,) and each blinker will blink properly.

Now, as soon as I turn the taillights on:

The "TAIL" LED Lights right up.. but if i step on the brake pedal OR turn either blinker on, none of the blinker lights light up on the tester.

This is where it gets good:

IF I plug my trailer in and turn my trucks headlights on and turn the right blinker on, my left REVERSE LIGHT will blink. Left blinker and the right reverse light lights.

I should also add that i have a tow connection in the front of my truck ( I assume it was, at one point, pulled by an RV by a previous owner and he/she used this connection to work the lights.) It is a 6 or 7 pin round connection (Just over an inch in diameter.)

My question is.. whats up with this? Could the front trailer wiring harness have anything to do with why my REVERSE Lights blink? Im semi-educated in trailer wiring, but i cant figure it out.

All Im going to do now is attempt to remove that front trailer hook up completely and then re-ground everything. My trailer is galvanized, but I ran a seperate ground wire out to each light with some tap splices and some tape, I think that should be sufficient. I've just never heard of the BACKUP LIGHTS blinking.

I assume you have a test light, so starting at the front connector, ring out the individual lamps using the ground on the connector. Then do the same at the back connector. If your problem isn't a corroded connector or ground fault at the two connectors, it's at the trailer and that's easy - just hook it up, pop the light covers and see what's what.

Problems like this are usually ground faults or corroded connectors or lamp socket problems, but you may want to check the fuses in the accessory panel under the hood of your truck.

There is a possibility that the wiring on the truck may not be compatible with your trailer - that RV thing may have been wired differently - you never know about RV dealerships. That's why ringing out the connections and matching them with your trailer is the best way to start.