soak plug as much as you can with penertrating spray,wire holding nut is 7mm socket ,heater plug is 10mm long reach socket ,if youve got aircon undo 4 bolts holding compressor and move out the way dont need to undo pipes ,careful you dont shear plug in half ,although 300s normally dont,if your unsure about one undoing try with engine hot ,shouldnt really need glow plugs on 300 unless really cold ,you can test them with jump leads when out of head by putting earth lead on plug spanner flats and touching positive lead on end were wire fastens if working they should glow red quite quickly