Almost certainly the wiper motor / relay control unit which is part of the wiper motor assembly. Part into at bottom of post.
From my notes:
28/11/19 : General
0048,980 Wiper intermittent wipe and sometimes slow wipe not working
Removed steering column cowels.
Tested switch by bypassing / hot wiring as suggested in Workshop Manual
Switch OK as same problem occurs
Fault lies in relay unit attatched to wiper motor
Solution for time being is to use Flip Wipe or put to Fast Wipe and then
back to Slow Wipe if required. Going straigh to Slow may not work. Also
after Fast to Slow to Intermittent then one may get two wipe cycles
Relay unit seems to be wired to be sequential type of control requiring
transition between states to get the correct wiper motions and correct self
parking.
Long term solution is fix or replace relay unit or bypass relay unit to
have just Flip, Slow, Slow, Fast or have Flip, Slow, Fast, Fast.
04/01/20 : General
0049,049 New Wiper Motor Unit - old one to stock for later refurbishment if possible
Note: When I removed the old motor the black plastic water shield was a folded bag
type of assembly and the bag when unwrapped was full of very wet dirty/old leaf
and other debris.
So on refitting the new motor I chose to cut the plastic shield to provide only
top down cover and to NOT enclose the motor unit
This is the original Fiat Part Info:
New Spare Part | Fiat | 9947465 | | | 0 | Barchetta | | Wiper Motor | Wiper Motor complete with Intermittent Switch Unit | | 28 November 2019 | | | | |
This is what I replaced with as original Fiat part is no longer available
New Spare Part | Magnetti Marelli | TGE428G/0643428070010 (Motor - 0643428070000 / 8-323-A) | 0 | Barchetta | | Wiper Motor | Wiper Motor complete with Intermittent Switch Unit | | 23 December 2019 | | | Fitted to Mirella | |