Problems with a garage wiring my car to 13pin

Hello all.

Just thought I would vent my fustrations and see what everyone else would do if they were in my position.

I have a VW Passat and it had a 12N socket fitted when I bought it. It is a RHD car but I am working over here in Germany. I took it to a VW garage and asked how much it would cost to convert to 13 pin. They quoted me €100 which I thought was pretty good. I booked the car in and a few days later they called and said it was all dont and they did it for €50? Alarm bells started ringing and when I got the car home I took a closer look at the plug. They had indeed fitted a 13 pin plug but only the original cables.

I took the car straight back and they said i dont need anything else... after a small arguement I told them to fit the complete system. They charged me the remaining €50 and I got the car home. I decided to have a look at the socket again and realised they had only added one cable. Back the car went this time with a wiring diagram and I told them I want the car done correctly. Again after a small dissgreement they took the car and a few days later I was called to collect. There seemed like there were more cables but as I have had so many problems I dont trust them any more and I took it to a friend who has a trailer plug test kit. he plugged it in and said it seems ok apart from the reverse light. So i unscrewed the plug and took a look guess what I found:

Pin 8 not wired in pin 11 not wired in and pin 12 ( I know pin 12 is not used and so am obviously not bothered about it).

I called the garage up and spoke directly to the boss he said its my caravan at fault lol... I told him my car is missing the wires its not my caravan and he wouldnt listen. I told him I will bring the car back and he can look himself and he said yeah fine and hung up. Very profesional.

What would you do? I am totally fustrated and I feel like doing it myself. But I dont see why I should as I have paid them to do the job.

Thanks In advance... Dave