You can use the existing wire *IF* you are willing to have the fan and light and heater all come on at the same time *every* time. But...I would look closely at the heater rating and see if the wire size is adequate. You'd have to calculate what other loads are already on that circuit too.
Most Heat/Vent/Light units are 1200-1500 watt heaters and the manufacturers state "Use on dedicated circuit only."
If you're gonna go through with this, now would be a good time to bring in 2
-20a circuits, 1 for the heater and the other for a new 20a bathroom GFCI outlet.
H-V-L units (w/o the "night light") require a 2-wire cable and a 3-wire cable to have seperate switches for each function.
I used a 12/2 for the heater on a spring-wound "Broan" timer, on it's own circuit, and a 14/3 from a local lighting circuit for the light & fan. The fan is also on a timer, and the light comes on with the vanity lights.
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