The AUSubaru.com.au Forums

if you happen to have the gearbox out of the l series take the time to check the speedo cable attachment.
it is a large nut which sounds good but unfortunately it is made out of plastic, and if it has been in for a while it is practically impossible to remove when in the car as the nut simply distorts so the spanner won't grip.
if you take the time while the gearbox is out to check the nust is removable so when you eventually have to repair a broken drive you can get it out.

I've got a spare if you need one:) or you could try and get a new one from subaru:p

but yeah, good tip:)