In most cars the RPM signal comes in the form of a digital signal, too. E.g. a block wave with one wave for each rotation (or a number of waves per rotation) - count the frequency to get to the RPM.
The dimmer could be an analog signal, also a digital one: look at PWM (pulse width modulation).
The complication you're left with is in the hardware, particularly how to protect your electronics from all the noise. That is actually not even all too complicated, it's just a bit of work to solder it down properly.