I'm kind of in the same boat with my car (not the z4 - that has now been sold).
I feel your pain maniac.
My story - bought new car in October, had mobile audio installers come to the house to fit fancy new all singing / dancing stereo. They left after a couple of hours & told the wife they had changed their minds about fitting it & wouldn't be doing it.
Came to drive the car next day - all hell broke loose / smoke in cabin / dash lights going mad. Car trailered back to dealer - turns out the loom has been damaged & is not salvageable. Still waiting to get the car back (hoping for next month).
Looking at a £12k bill which I will have to stump up to get the car back.
No chance of getting the money back from the audio installer - they point blank deny ever having seen the car, never mind touching it & suggest unless I can prove otherwise I should F*ck right off. Court is a non-starter, they laughed in my face - they do not have any assets in the business to take.
Wish you better luck!
Matt