I'd suspect the switches in the doors over the relay box. Since the seats move back when you recall a memory position, the relay box works (or else the seats still wouldn't move back). Not familiar with the R129, but if they're set up like the 126/124 switches, there's a ton of little parts inside that all have to move together to give the proper function. The grease they used inside gums up and prevents contacts from making or moving properly.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]