I assume it still goes in reverse
In which case the part that lets the car know its in reverse is knackered.
Dont know whether the auto boxes have reverse light switches, because thats what i would say is faulty if it was a manual.
Is the inhibitor stwitch part of the gearchange lever area? I'm thinking if there is a seperate sensor that senses you are in reverse etc, and then relays this to the inhibitor switch, that sensor is the problem. Does it start in drive as well?
Sorry if this isnt much help, dont know much about the autos
If you have the Sports auto box it could be an e.c.u problem mate, an easy swap for fault finding.
otherwise on the standard auto sounds like a selector or box problem as you have changed the inhibitor already!!
Good luck fella.
Nick..