Your 850 setup may be much different than mine, but I placed my gauge forward of the shifter in the bay where the 'ashtray' normally is located. I took my lighting leads from the ashtray light. To color the lighting to match my instrumentation, I cut the fingertip off a disposable green vinyl painter's glove (Home Depot) and fit it over the lightbulb, secured by a rubber band. It's a perfect match.

Getting through the firewall is no big deal, but getting to the instrument cluster is a pain. You might get to the internal line from underneath? If you run a new line from the engine you'll have to be creative to make sure you have a good connection on the manifold vacuum tree.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)