Dennis Harms interview at the 30th anniversary of the HP 12C

[Steve Simpkin]

While I do not know where to find the exact video you mentioned, here is a similar presentation that Dennis Harms gave at HHC 2011.
Note the following may not work on a mobile device.

Go to https://videos.hpcalc.org/

On the left side, scroll down to the following section:
HHC 2011
Back to the Future

Click on the link on item 5. It is labeled as follows:
5. Dennis Harms
HP 12C - Why has it lasted 30 years?