After looking on some literature about the antitheft system used in mercedes I noticed that was an auxillary battery needed/installed. When I went out to the car to find it I couldnt find it. Was this item optional or where other possible locations of it?

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)