"SES" = Service Engine Soon/Check Engine.
I wasn't familiar with that era
B body, but the fact that the lights are in the center of the dash and there's an aftermarket stereo beneath it might be a related issue as someone mentioned earlier. All depends on how it was installed.
In the Regal, your indicator lights are on either side of the speedometer.
I can't read it in the video, but you should be jumping the ports labeled "B" and "A" if the letters are F-A, left to right. Should flash once, then twice, indicating code 12, pause for a few seconds, then repeat twice for a total of three times. After the third code 12 flash, it'll go through actual trouble codes if there are any. Then it will start over with code 12. All from Chilton's manual and this website:
https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php