Try it and see if it works. But a horn is a fairly large DC load- 5 amps or better and causes lots of interference when it goes off due to the magnetic actuator in the horn which results in lots of voltage spikes. Your MFD may not be happy with the resulting voltage drop and spikes.
But you are going to need to run a shield ground back to the ground battery buss anyway. Why not run a new ground at the same time for the horn/VHF and use the two conductor cable strictly for your MFD. That will give it clean power.
David
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