If you are live in a strata (ie you all put in levies to take care of common property) then the Executive Committee and Owners Corporation would be responsible for ensuring by laws are observed. Everyone who owns in strata make up the Owners Corporation. Also, individual owners can take other owners to task regarding compliance with by laws.
I would not be putting something on common property because someone else is. They could take you to task over this garden. Their junk car is not a permanent fixture ie something that can be moved easily even if your neighbours aren't. Don't be putting up a raised garden bed with stakes into the ground securing it. Could be viewed as a permanent structure. If they object to the garden bed and complain, they can have their car towed away overnight and leave you with the permanent fixture. They win! Why not put pots along this area. They, just like the car, are not permanent. Someone gets picky, you can move the pots overnight. Otherwise, do the right thing, even if your neighbours don't and get permission to build this garden bed.
If you do something because your neighbours have, then the other neighbours might do the same and it may not be something as nice as a raised garden bed. And then the next neighbour might do something else then you end up with a hodge podge look, very unappealing. If this car is on common property, about time someone does something about it before someone gets injured due to the car being there and sues your strata for injury/damages.