In case you didn't realize the "Mo" in "Motown" stood for "motor" as in "motor cars" (and not Moravian, since Detroit's Moravian population is actually surprisingly small), this 1965 video should be clear everything up. It was shot for Martha Reeves and the Vandella's song "Nowhere to Run" and yes, it's entirely shot on the fully operational Mustang Assembly line at the Dearborn Assembly Plant, part of Ford's River Rouge complex.
This probably couldn't happen today. A music video shot in a fully running automobile assembly line, with no special safety equipment or precautions taken at all. Hell, when I visited Nissan's factory I had to wear, aside from safety glasses and hard hat, special extra sleeves that they gave me to cover my dangerously exposed elbows. The idea of three women in mini-skirts jauntily traipsing through partially-finished cars as they trundled down the line would probably cause modern safety inspectors to void themselves with lavish abandon.
Based on clips from a Motown documentary, it sounds like the workers weren't even that prepared for the invasion of the singers, with employees grumpily yelling at the women. Mustangs were in such high demand at the time there was no way Ford was going to stop that line even for a second, no matter how many musical groups clambered in and out of the unfinished cars.
It also seems this is one of the first music videos, period, so that's compelling as well. Still, the real treat here is seeing such a candid look at car manufacturing; in light of today's highly automated factories, it's striking to see how much work was still done by hand.
Also, if you're looking to sell a 1965 white Mustang convertible, you may want to make the claim that yours was the car in the video. That's gotta be worth at least an extra $200 on Craigslist.
(Thanks, Galen! Sources: Dutch Mustangs, Google Books)