The aft shower unit in my SM had developed a small leak.  I found this thread when researching how to open it up.  However, the Grohe shower unit in my SM is a little different from the one in the link that Alan posted.  Mine does not have a screw holding the ON/OFF cap in place.  It is just pressed on so I had to use a screwdriver to gently pry it off.  Upon disassembly I found that the ON/OFF cartridge had cracked.  There were some numbers engraved on the cracked cartridge but none of them appeared to be the part number.  I used Google images to find the closest visual match.  I got lucky and guessed right. The part number for the ON/OFF cartridge for my Grohe shower unit is 45882000.  I found them on Amazon for about $20 but they also have them at Home Depot for the same price:  https://www.homedepot.com/p/GROHE-Ceramic-Headpart-45882000/204193498.