Well I'm slowly getting there.
Still not working with my Xperia Z - but this is where I am at.
I have managed to activate mirror link on my phone by going through the settings - Xperia - start mirrorlink option.
When connected via USB - the phone connects without any error to the car , the mirror link front end pops up on the phones screen and even a steering wheel icon apears at the top indicating successfull connection.
However - frustratingly - the option on the lexus menu is still greyed out.
I must be getting close surely!
Any advice ???