Cheers Matty!
Continued.
Explanation from ELSA about Display Group 19, Display zone 4, which sheds some light on specifics of the protocol of the yellow/black wire that connects E87 (connector D, contact 11) with J505.
* "0" or "00" is displayed if the vehicle does not have a heated windscreen or control unit -J505.
* The heated windscreen control unit -J505 switches approx. battery voltage to the output to the -E87 as long as the voltage at the -J505 is greater than 12.70 V and the control unit is not too hot.
* If the windscreen heating is to be switched on, the operating and display unit -E87 actuates a transistor which reduces the voltage at connector D, contact 11 by 50%. The control unit -J505 switches on the heated windscreen.
* If the voltage (vehicle voltage at -J505) falls below 12.70 V while the windscreen is being heated or if the control unit -J505 becomes too hot, a switch in the -J505 opens and the voltage at the output to the -E87 falls below 5V. At the same time the heated windscreen is switched off by the -J505.
That hopefully is aligned with Matty's content.
There is a lot of further reading if you google for "E87 J505"