Left rear? Perfect, you can match the passenger side easily*
Remove bumper (4 crusty m8 nuts underneath plastic covers, covers held on by sheet metal screws)
Remove bumper shock (one large bolt inside spare wheel compartment)
Drill hole into bumper shock (wear a face shield!)
Let all fluid out
Pound on it with a sledgehammer until it is the same length as the other side.
*To remove the driver's side bumper shock, you need to remove the fuel tank

Extremely dumb Fiat design, they could've just rotated the fasteners 180 degrees.