Backup Tach

I am putting in a backup altimeter, airspeed, D1, GPS, and an electronic tach. Been studying the wiring diagrams and am unsure where to place the signal wire and ground wire for the tach, and whether these would interfere with the tach in the skyview display. Can one tee off the RPM and RPM ground wires ID'd in the wiring diagrams or do you have to string wires to the engine compartment? Any one tried this? Any advice welcome. RPM not necessary in a SV screen failure but might be nice but not worth making a mistake over.

Doug Dahl 80 hrs on my RV-12

I haven't tried this Doug but I got some time to play with the RPM line from the engine compartment as my RPM indication in the SV was going nuts at higher RPMs and jumped all over the place. I learned that Van's had reduced the value of resistor in this line (not sure where that resistor is located, might be in the clamp shell or in the black box). I added an additional resistor in line in front of the firewall to calm down the RPM indicator. So be aware that if you pick up the RPM signal from the wire as it comes through the firewall (which is my recommendation) you probably need to add a significant resistance to calm down your indicator (I'd guess 100-120 kOhms). The signal from the Rotax is a voltage pulse and the SV display does not put a load on this signal, so I don't see why you couldn't split the signal off for a second indicator.
I don't have access to the schematics right now but I could look up where that wire is going behind the firewall if you need help with that.