A friend of mine has an Alfa T-Spark not sure what model - two door
thing that looks like vaguely like a Honda Civic to me and it's about
five years old

The problem is that one of the rear lights is on all the time and she
has to take the bulb out as it's the only way of not having the light
on. It's a light as opposed to a brake light.

Any ideas on what might cause this please and any suggestions as to
how to cure it?

Many thanks