The problem part is the motor itself, that is if your 156 is the same as any of my 3?
the motor itself, (although i believe there may be two types, so check first) has within its top section where the incoming cable connections are housed, three brass rings, a bundle of contact grease, and three brass contact arms.
the arms tire, and the little contact ball at the end of the arms becomes worn too, so a poor contact results, and it is usually the park/intermittent ring which is the same one, that tires first, leaving you with the remaining circuits or circuit just for a continuous speed or two.
some folk claim to have repaired theirs by cleaning, bending, soldering more onto the ball etc etc but a new motor complete with contacts is about £50/60 i think.
try shop4parts, which is where i get a lot of my alfa parts from, theyre good at delivery too, but remember to check the motor layout/pattern first!
if you're lucky enough to have the type i had and bought, its a doddle to replace too.
wiper arms off, platsic scuttle panel off, split motor from linkage, take note of the linkage and arm positions can help, unplug and unbolt, remove, replace, and reverse to rebuild.
nothing techy and no special tools.