Are these side marker lights, which illuminate when vehicle side & rear lights are switched on? If yes then command signal will probable be taken from the Fiat concerters panel, at the base of the RHS B pillar (behind driver if RHS drive vehicle). As suggested by
@rayc , the side marker lights will be a Hobby installation. I would expect a relay box to be concealed somewhere. I have read reports of similar boxes being hidden behind a wardrobe, or under a front seat.
Again as suggested by rayc, the electrical installation in your Hobby is probably manufactures by Schaudt, who supply adapter boxes AD01 or AD02, which use the Fiat control signal to switch the marker lights.
I am attach details of the Fiat panel, and the Schaudt boxes.