Here is some advice from AlfaDriver.com, where more tips and fixes are available.
Rear door issues
Happened out of the blue last week – all other doors working fine. first time it happened, it wouldn’t open from the outside but my daughter opened it from the inside, since then it won’t open from inside or out. I suspect a rod has disconnected itself inside the door or the locking motor is jammed, seen both occur on a few 159s.
Selespeed gearbox stuck in neutral
Just changed my selespeed pump and brake switch two days ago and all is fine, then today while I was slowing down at traffic lights the gear suddenly dropped to neutral I couldn’t engage any gear at all and check gearbox message appeared. I tried to restart car and still the same meaning I was unable to engage reverse or gear 1.
In the mean time, disconnect and reconnect every plug you can find on the selespeed loom. Then the next logical thing which would prevent the gears from engaging would be the clutch speed sensor (55185789), though if that fails it would normally still be attempting to change gear but just be a bit slippy as it released, which means the problem could be to do with the Selespeed clutch bearing (Slave cylinder) fail and the selespeed actuator clutch pump oil seal being broken.