To prevent moisture from entering the electrical cable when disconnected `_________`.

A Hang it with the plug down.

explanation

STEP 5: Disconnect Air Lines and Electrical Cable
Disconnect air lines from trailer. Connect air line glad hands to dummy couplers at back of cab or couple them together.
Hang electrical cable with plug down to prevent moisture from entering it.
Make sure lines are supported so they will not be damaged while driving the tractor.