As DKLawson says, the steering column should be earthed naturally.

But I've found this doesn't always work well so I run a black wire from the indicator stalk clamp to a good earth on the parcel shelf.

Start with your car's wiring diagram in front of you and also see the thread linked below.
//mk1-forum.net/viewtopic.php?t=7706

Don't have a wiring diagram?  Try the one linked below.
https://www.minimania.com/images/wiring/PDFs/wiring01.pdf

The purple/black wire is the horn's ground connection.  It goes from the horn to the 5-pin turn signal switch connector.  It continues to the switch itself where it is connected to a wiper arm that rides on the brass slip ring.  Through the slip ring the purple black wire connects to the spring loaded contact in the center of the column.  That spring loaded contact pushes against the back of the horn push button.  When you press the horn button, it connects that center contact to the steering column... completing the horn's ground circuit and sounding the horn.  I don't remember how the column gets its ground connection... but it is grounded.

The thread linked above shows how inside the column a second short length of purple/black wire is INSIDE the column and goes from the slip ring to the spring loaded contact.

Last December, I put a harness in a MK 1 and redid the horn push.   I don't remember but I think the horn wire comes in to the signal stalk. I remember wondering where the purple wire attaches also. The short bit from the signal stalk has a wire that is not purple in mine.  I connected all the other  matching colors and the purple/black went to the last stray wire. Car is gone now, so I can't look. Maybe that will help?

Having horn problems getting horn to work.Replaced sterring rack last fall,dont remember where the wire from the horn attach to the colume.The wire that attaches the spring to the c clip is there,but don't know where the purple black wire from horn attaches.Any help would be very grateful.