Weak turn signal on trailer plug in on a 2006 dodge 3/4 ton diesel?

I've put a test light to the plug in and the left turn signal is alot dimmer that the right turn signal. Any ideas on what it could be? And if the flash relay is a possiblity tell me where the relay is.
ok i unpluged the conector behind the the plug in itself the plug looks good but it is still dim. so it has to be before the plug in. and where is the ground wire for that?

The lt and rt turn signal operate off the same flasher unit.Most likely is a corroded connection inside the trlr plug. As for locating the flasher, simply listen for the click, click under the dash, thats the flasher/relay.