https://www.youtube.com/watch?v=4mnkAFdwY0o

A useful video that details brand extension, giving a case example through the Ford and examining the challenges that might be faced in extending a brand,

https://www.youtube.com/watch?v=svMrZ4ms_5w

This short video from the Kellogg School of Management outlines some of the concepts that need to be engaged with when managing a brand portfolio.