I am pulling my H14 with a 1989 Nissan pickup. I also have the same problem. On my trailer I modified the lights, removing the wire on the frame bolt (for ground) and soldered on a separate ground wire. I then ran this wire all the way to the hitch plug. This bypassed using the frame for any electric connections, guaranteeing that I will never have a rusty ground connection, the largest problem for all trailer wiring. With this perfect ground, I still have the same problem you do, my flashers are very dim. I am using a conversion kit that came from Napa on my truck. I know this is not the answer you are looking for but it eliminates a bad ground connection off the list of possible problems. Please keep us posted once you get it fixed.

Thanks
Capt Morse