The HU unit comes out with a set of keys (you can pick them up on ebay for a few quid). For memory, the top plate is held in place in four places (will be obvious when have the HU out). Just level the plate off with a flat head screwdriver. I managed to feed the cable out the back corner. The gap is a little smaller than the cable, but doesn't affect anything. The cable is easy to feed through to almost anywhere, although can be a bit of a fiddle.