The harness is mostly for the turbo timer feature. All it needs for the boost gauge is switched power and connections to the pressure transducer it comes with.

www.alamomotorsports.com has decent prices.

Sixto
91 300SE
87 300SDL
83 300SD

I think I am going to go with the one I linked to when they get re-stocked. It is smaller 1" x 2", and in psi. I hate metric.

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K