Just fixed the sword (fan control) on my dad's '90 535. Selected a far more robust MOSFET from International Rectifier: IRF1310N - Digi Key stock at about $2 each.

Also installed a 40V TVS diode from my parts bin across the FETs to protect them from surges - likely what kills them in the first place.

On re-assembly it still didn't work. The problem was the comparator (8 pin chip near the relay). Used an ST LM2903N.

All good. $9 and a bit of time vs paying through the nose.