A female headphone jack on the back of the control box is not always convenient in practice. The only thing worse than this one is an output jack on the side of the housing. The perfect place is when positioned on the shaft. See the photo report: a headphone jack at the top end of the shaft. The waterproofing quality has remained constant.
A hunter did a simple enough thing… He made a hole in the shaft. Inserted a rubber grommet into that hole (to prevent the cable from being damaged by the hole edges). Cut off a piece of the headphone cable for Garrett AT with a connector. Inserted the latter into the headphone jack socket and made the cable tail go inside the shaft.
A waterproof connector is added to the cut off headphone cable. The counterpart of this connector is fastened under the armrest on the shaft (the cable inside the shaft is attached to it).
Waterproof connectors are sold separately and there is a wide range of them. As an example: 523-12-08BFFA-SL8001 and 523-12-08BMMA-SL8001.
Wired headphones are not only thing that can be conveniently used with such a jack. Alternatively, it makes attaching a wireless headphone transmitter box to the armrest easier.
Read about wireless headphones for Garrett AT metal detector (with jack for Garrett AT PRO/Gold) here. Deteknix has rolled out a wireless kit.