Have you checked
a) the button on the end of the stalk actually is earthed to the stalk so when the button touches the point termination of the brown wire the wire is earthed.
b) the stalk is earthed (ie metal to metal contact with stalk unit to the steering column metalwork.
all the above can be checked by seeing if the brown wire is earthed when the button is pressed. The Lancia installation is to switch the earth (-ve) to the pump (live being fused supply)
You may wish to check that the switch installed is not switching the pump live......
Thanks for this, which of course makes sense. I'll check this out tomorrow.