Most of the wiring for the 72/3 280SEL 4.5 would be same as on a 72/73 R/C107. The wiring diagrams are in the chassis section of the shop manual on the Startek sire under Workshop Resources.
This 72 350SL diagram would be close:
https://dl.dropboxusercontent.com/u/54783344/350SL_Wiring.pdf
Section 07.4 of the M116/7 manual has greater detail on the injection harness wiring.
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Graham
85 300D
,72 350SL, 98 E320, Outback 2.5