Just installed a manual boost pressure control valve on my S-350 and I want to install a boost guage in the ashtray where I can check boost pressure when I want to. I have just about ruled out trying to route a hose through the firewall, but I have found a boost pressure guage that has a transducer and transmits its data over a regular wire. I know there is bound to be a wire going through the firewall that is not used by anything in my car. Problem is finding one. Can anybody help me in this area. I know the S-500 and S-600 have more features and possible wires, so my S-350 possible would have one unused.
Thanks!

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