Have you checked the Owners Manual? The manual for my much older x244 lists fuse F63 for "Ambulance rotating light". Fuse F63 was located in the RH (optional equipment) cab fusebox, but was this not deleted on later models?
I cannot see why a relay would be required for a relatively low current device.
The beacons will not have been fitted by Fiat, so the wiring could be anywhere. Hopefully taken from the fusebox, but could be spliced off of anything that is live at the right time.
As said above, have a look at fuse 63. If not successful, is there anything else not working that might point you to the circuit used. Look for added wires at the fusebox.
Failing all that, the beacons are on the roof, the wiring has to come down somewhere, likely to be the screen pillars. Now you need to grovel under the dash to look for the added wires, and follow them.
Just thought. If it is a 2022 model, it is very recent. Ask the supplier of the vehicle, or the fitter of the beacons. There must be a warranty on the workmanship and parts, so go back and get them to fix it - properly.