I've added another feature to the secondary holder.
Since the mirror is siliconed back onto the holder, it's not easily removed (unless you really have to)
If the setting on the heater has to be changed, the mirror would have to be removed.
So I decided to make an inspection- adjustment port on the holder.
To stop the heat from escaping, the hole is easily covered using a wiring plug.
Now I can adjust the heat setting easily.
