I have an '86 505 sedan with central locking; it locks the trunk, the doors
and petrol door.
As of yet I have had no problem with the trunk not locking. You might what
to try finding a latch for the trunk of the sedan to compare with the latch
of the estate. It seems to me that they would be similar with the exception
of the placement of the button/key device. It might be worth it.
-Adam Recktenwald