Ive done both in my zx2. Both are cyberdyne electronic gauges.
I replaced the stock sensor(actually a trigger) The car does not need it and it wont throw a code or anything if its not there. I ran the wire into the cabint o my gauge. The oil pressure unit also had a power, ground, and illuminate wire. I ran the power and ground to headunit wiring harness and the illuminate wire to the dimm wire on my headunit harness as well.
Volt was just power, ground, and illum. Which I ran all to the same as the oil.
Basically what the illum does on the cyberdyne is when your headlights are on it dimms them so it doesnt effect your line of site. It also is the default switch. If you turn the car on with the headlights already on it will tell you what your last runs high oil pressure/volt and low oil pressure/volt was.
Other zx2 is getting quite a few extra gauges. EGT pyrometer, wideband AF, Volt, Oil Press, Air Temp, and Boost/Vacuum