an old brass cleaning brush chucked up in a drill will do the job nicely. a 22 cal or 6mm should fit.
Hooked my utility trailer up today to my flat 4 connector and had to plug it in twice to get running lights to operate. It did the same when I rented a Uhaul trailer a few weeks back. Anyone know if they make plug connector cleaners like a battery post cleaner? With male and female ends with wire bristles.