From memory it is called a 'T2' tester.
The T2 test plug is located on the left side of the plenum chamber
near the battery on a RHD car.
Is there a resistance value for the rotor button from centre to tip?......
Yes, this would be the 'T2' tester (named after the connector,
which is marked as T2 on the Motronic injection schematics).
That location a bit strange to me, i thought only the ABS diagnostic
connector is located between the intermediate bulkhead and the
passenger compartment (the ABS unit is also capable to self diagnostics).
But if the wire colors are match there's no doubt.
In the factory rotor button there's a 1kOhm resistor between
the centre contact and the arm tip. From some aftermarket ones
this resistor is omitted.
I've attached the timeline of the ECU diagnostics.
Note: by pressing the pushbutton switch more than 10 seconds
during flashing a stored error, the flashing process will return to
the first stored error. When the pushbutton is pressed more than
10 seconds during the "no more errors" indication the fault memory
will be erased.
You can move to the next stored error by pressing the pushbutton
to 2.5 to 5 seconds after a flashing sequence is completed.
(I assume you already have the flash code table.)