I have a '91 Admiral 27" TV. I purchased it used many years ago and could never figure out how to get it to switch to the video input. Anyone know where I could find some info?

Am not familiar with this particular model but sometimes tuning the channel below channel 2, or entering channel 91 will enable the video input. Also have you checked all the menus for a command to enable the video input?
On some RCA sets I have needed to have the remote control to activate the video input.