Your problem looks like one I have allready had. If the 50amp fuse blowns. you have a short cut somewhere and still in place. You have to find where he is. It can be long and difficult. In my case, it was just a small wire burnt in the immobiliser device. Desperate to find the problem we asked some help by phone to MMC. They asked to check the immobiliser and to try without it by switching to a non immobiliser plug in taken from a 3W in the showroom without this option. Miracle everything was operating again. Just a wire to replace and the immobiliser was ok again.
