[Robznob11]

Ok I think I may know whats going on here ( I build led tails and have had the same problem) yes you are correct that 1157 only go in one way....but gm did not wire every truck the exact same. Andy will vouch for this because he had to swap the wires on his tails. One wire is park positive, one is blinker positive, it gets the ground from the housing. If the wires on the 1157 socket are reversed then you will turn your park light to the on position, but insted of the dim side lighting up you have the bright side light up. Now when you hit the blinker (which should be hooked to the bright side) it is triggering the dim side to flash but it can not do so because the park circuit is already supplying power to the bright side.

Led turn signals work using a single led (many in a group) to run at a reduced brightness for park and full power to get brighter for the blink. Unless you have more expensive ones that usea seperate circuit to kill the park side much like a switch back. Most manufacturers take the cheaper route because for one its cheaper and 2 less circuitry to fail. There are numerous threads about led bulbs not all being the same as well dont assume because they are packages as 1157 that the polarity is correct for every vehicle its just not the case.
Hope this gets you up and going.
Also did you just reverse the flasher wires or did you confirm with a test light that X is recieving power from the fuse box. It would suck to swap wires only to find out a previous attemt to cure the issue actually compounded the problem.

Also before you cut the wires and try and swap them on the led turn the parks on and see how bright they are. Then pull the led out touch the metal end to ground and use a wire to touch power to each soulder nub separately. This will show you the difference between the bright side and dim side if the bright side looks the same as what you see when its plugged in and on park lights you have confirmed they need swapped��