headlight problem

[modifiedGMC]

I got my relays mounted and ready to hook to the headlights BUT I got a slight problem first. I got a 69 GMC with four headlights, halogens to be specific, when I have them on bright all is just fine. When I put them on dim the two outer light are on like they are supposed to be but not as bright as they should be and the two inner lights can be seen barely glowing when they shouldn't be. I thought bad grounds , so I took the grounds loose and cleaned them to the bare metal... Didn't help. so I tried cleaning the terminals on the sockets and bulbs... didn't help. BUT while I had one of the inner bulbs out I took a piece of wire and stuck it down in the socket on the ground side then touched it to ground with the dim lights on .... no help. Then I took that same piece of wire and touched it from the ground side of the socket over to the other side of the socket and the two dim lights got brighter like they should be and the other dim light wasn't glowing. Does any body have an idea what could cause this? should I go ahead and wire up the relays and take a 50/50 chance that it could solve the problem?