Right rear turn signal

i have a 1994 ford ranger .
the problem is with the right rear turn signal. it works fine except when i step on the brake. at that time, it turns dim and will not flash.
all of the other turn signals are not affected like this.
i have checked the bulbs and they are all good. i mean the three in the rear tail lite.

Dim lights often mean they are not grounded properly. Make sure all parts are present and fastened firmly to the truck’s body or frame (on the metal, not plastic).

Agreed that this is a grounding issue. Locate the ground point, clean the connection and re-tighten.