Trailer wiring problem checked/replaced almost everything?

well to start off i havea 6x9 john deere trailer or somthing like that, its a lil older like 10 year and not sure how old the wire is but i pluged it in and i didnt see the lights working so i replaced all the wireing on the trailer and sanded down the ground to bear steel the bolt i sanded and much as i could but didnt get all of it but it still smashed on the bear steel, and its grounded by where you link up, well the old factory lights that are mounted in the fram i took off the lenses and they were bearly lighting up and only doin it when the turn signals or brake were on not when just my lights were on so i replaced the lights, one worked with turn signals and brake so i was liek w.e that shall work for now hooked the other light in and nothing was working and the park in my car has lil lights on it to show it working and they blink the u have turn signals on for what side and one that turn on when ur lights are on and it all seems to be working and i have a good tester not the one with just a bulb but one that give reading and stuff im not sure what setting i had it one but when i checked the ground (place the black where i have my ground wire and and red on a trailer power it read 0.75 and when i tested the power and ground to the lights i got 0.75 when brakes where pushed and it would go from0.00 to 0.75 and back to zero when the blinkers were on, and i tested the lights on my car battery they light up and everything just fine and arnt dim so what could be the problem i pretty much did everything and seem like everythings workin fine but nothing is happening