I would highly advise against a trunk-mount rack.
All trunk mount racks depend on a tie to the bottom of the trunk lip to prevent launching the rack over bumps in the road.
1) the bottom of the trunk lid is above the license plate. Non-owners, look up a rear profile to see how high this is in comparison to most
2) the bottom of the trunk lid is
PLASTIC over metal (at least on the Abarth). The tie down of the bike rack will stand a chance of tearing off the plastic cladding.
3) if wrapping over the bumper to the under-side of the car for tie-downs, you'll need neoprene under the straps to avoid nasty near-vertical lines on the rear bumper from strap vibration & abrasion.
Previous discussion
Note: the Saris bones rack referenced is not recommended for use by Saris - they say they don't sell anything that fits the 124.