I have one I hard-wired into the Jetta. It comes on with the key in the ACC position, and has batteries as well. The display has started to fade a bit, finally, so I might just have to pick one of these up. The indoor/outdoor temp feature IS pretty handy! I found that the best place to mount the temp sensor outside is right where Mercedes mounted theirs -- on the front bumper, behind and a bit under the license plate. Mien has been pretty accurate!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)