Easy way to do this is two diodes. Common is the clock in.

One from the master switch hot; the other from the avionics hot.

When the master is off, that 13.8v supplies the clock. When it's
on, the avionics bus is hot.

No wire-stringing needed.
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433