Vauxhall Corsa Owner's Manual page 154

Insert coupling ball bar into housing of
coupling using a little pressure until it
audibly engages.
152
Pull coupling ball bar to check that it is
engaged.
If coupling ball bar is correctly engaged
z the red field on release lever 2 will not
be visible,
z and release lever 2 can be pushed no
more than 5 mm toward the coupling
ball bar.
Repeat procedure if coupling ball bar does
not engage.
Press red button. Put on protective cap and
engage.