"In August 1989, Pam Dawber, Joel Brooks, David Naughton and Jenny O'Hara reunited to film a public service announcement for the Center to Prevent Handgun Violence in Schaeffer's honor."
I have been very interested in Rebecca Schaeffer's case for quite some time, and would very much like to see this. I did find at least what may be a partial recording of it at around 0:51...
According to this article, the PSA was being shown on the MovieTime cable channel which is now E!. It also mentions the possibility of it being shown on the USA Network, which was running "My Sister Sam" reruns at the time.
I have also scoured through 1989 USA Network commercial break compilations around the same time the PSA might have been shown, but have come up with nothing. And there's basically no footage around from MovieTime, so I'm not sure what to do next...