When I typed in the Bosch part number, one of the first search returns was this thread, which coincidentally also refers to an '85 w123... so I am not the first boy scout in the troop to have charging issues with this relay retrofitted, it appears:
Bosch glow plug relay: working? broken? stupid? (pick 2)
Also if I understood some of other info in the search returns, this relay was supposed be used with an alternate GP part #. This I will have to look at more, and as I have been swamped today with work I can't get to it just yet.