To add my own experience and a caveat:
The symptom my car(89 535)exhibited was the fan running any time car was on. Seemed to be speed #3 as turning to 4 increased speed slightly. Autopsy revealed 3 of the 4 MOSFETS were bad.
I ordered BUZ11a MOSFETs as I read they are more robust.
10 ST Microelectronics brand ordered from Ebay seller giorgio11185. Good thing he sells by 10 as three(perhaps 5) were bad. I only found this out after performing the repair and having only high speed function.
After dismantling again the MOSFETs were tested and two of the four installed were bad. Another from the pkg was also bad and two others gave odd results.
SO, given the tedious nature of this job I advise that these always be tested BEFORE installing.
After the second repair (all four tested BEFORE soldering in) the FSU is working as intended. Too soon to comment on durability of these MOSFETs but given the rate of defect out of the gate I don't recommend buying them.
Thank you for all the info and advice previously posted by others.
info by ross1