A 3.5 mm chassis terminal is mounted in the side of the front half of the scanner. The broad PCB track which is visible on the picture is connected to ground. After removing some of the protective cover,
the ground lug of the chassis terminal is soldered to this track. The other lug is connected to the wire running from the 10 k resistor.
Because the side of the scanner is quite thick, the chassis part can't be fixed on the outside. For some extra support, the terminal can be glued to the inside of the scanner with a glue gun.
After this modification, I had to reset the scanners. Pressing 2, 9 and Manual while turning the scanner on does the trick.