I would be hesitant to take the signal from the intake manifold; especially if you are ever going to add an intercooler. You want the wastegate actuator to "see" the pressure the turbo is putting out as quickly as possible, the farther down-stream it is the longer it will take the wastegate to react. The reaction time will be especially slow if there is a whole intercooler that needs to be pressurized before the wastegate reacts.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine