After gaining some experience I recommend to use a voltage level shifter instead of the resistor based voltage divider, please see here
The AC provides the signals via the CNS connector. It has 5 pins with a pitch of 2 mm (0.079''). It is out of the PH series from JST. The position of the connector is visible on the following photo of the indoor unit PCB.
Opening of the indoor unit should be done by a qualified professional because faulty handling may cause leakage of water, electric shock or fire!
The connector provides +12V. The DC-DC converter TSR 1-2450 is used to convert it to +5V.
The Eagle files (schematic & PCB) for the testboard are located in the sub-folder eagle. On the PCB are located many jumper and test pins to enable simple connections to measuring devices.
The following photo shows the assembled testboard.
The upper colored cables are connected to an AC remote control. The JST connector is connected to the MHI AC.
SCK is connected via a voltage divider with D5 (GPIO14) and MOSI is connected via a voltage divider with D7 (GPIO13) of ESP8266.