Make sure you solder the resistor inline with the POSITIVE (longer) end, or anode, to hook it up to standard 12v power.
Also, don't bother trying to get the film off the back of the gauge faces. I tried doing that by sanding the film off and only did the tachometer before I gave up because 1) the speedometer needle wasn't coming off, and 2) it doesn't make much of a difference. The light shines through only marginally brighter. Save yourself the time & effort, and the fact that it's damn near impossible to get your tach needle back on accurately so it reads the exact RPM.