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After experiencing intermittent problems with the powersteering in the 525tds, conclusive testing showed the Servotronic control module being at fault.
The unit is located in the left front footwell, behind the loudspeaker.
In my case the transistor had gotten so hot that it had lifted the trace off of the PCB, until it broke.
The blown transistor is a 2SB1203, datasheet can be found here.
I replaced mine with a TO220 version and pulled a wire through a hole in the PCB to connect the collector to the corresponding pin on the back.