Did Boeing really use ferrets to run wires through aircraft during construction?
No, Boeing didn't do that.
I asked their assembly plant tour
to confirm or deny this, when I found
a report that around 2010 some visitors heard this from a tour guide.  On 2020 Nov 3 they replied:

Hello,
Thank you for contacting us with your inquiry. This is a popular urban myth, there are no records or any other evidence that this was done at Boeing.
Thank you again for your interest.