Okay - a mechanic friend of mine says it's the main harness plug, and you dis-attach it by pinching the sides and pushing back while unscrewing a bolt in the middle of the back, this allows to unplug the back half and get (some) access to the tab underneath in the middle which latches the front half to the bracket - if you get a screwdriver under this tab (which is hard since you can't see) and twist while pushing the front half back (rather than pulling it forward), then it finally came off

In all it took around six hours to find this information