at the back of the unit theres a little hole that takes a thread (mine was m5) but they may differ.
i turned the supplied bolt into a hook using some 2-3mm diameter solid core steel wire and then hooked a zip tie through this and over a wiring loom hook on the heater unit almost straight behind it. this is sufficient in my experience to hold it from sliding forwards
now on my headunit the surround is one piece with the unit so it cannot slip back past the black surround, but when i fitted brad's newer headunit this was not the case, but sliding the supplied cage in did the job fine
its a little fiddly, but simple and well tested, my headunits have stayed where they belong for 3 years
