If your '90 LeSabre is similar to my '87 (body style is the same, and I assume the gauges are, although there were several available gauge clusters), the Low Fuel light should have a bulb - as the tank runs low, the light gradually lights up (sometimes very dim when it'* just on the edge of Empty, gets brighter as the level drops).
Security and Low Washer Fluid for me were empty - no bulb sockets at all. I don't have the security system, not sure if the Low Washer Fluid level was an option or was not available for '87 but was used on other years.
Make sure you change them all while you're in there... Since it'* such a pain the butt. I changed several burned out bulbs, and less than 3 days later one of the new bulbs burned out (behind the right hand side of the tach)... I was not in the mood to pull the entire dash apart to change it again!
As far as the green box with the speaker-like opening, I concur with Justin, it'* most likely the chime module. In my car, it'* mounted behind the wide strip of plastic on the dash that the vents are installed in. But it'* definitely possible in your car it is in a different place. GM often mounted these boxes under the dash in the area of the fuse panel or brake pedal. There are two different chime modules that I am familiar with on GMs, one actually uses a speaker, is the larger of the two, and is connected with the seat belt system and receives a signal from the Vehicle Speed Sensor in the transmission. The other uses a piezo device (which will look shiny, or shiny and rusty, if you look through the speaker holes).
HINT: If you ever want to/need to replace the piezo type, you can use ANY piezo chime module from ANY GM, it'* all the same part, maybe a different sound, from the 70s-90s as far as I know. I have used a chime module from a '91 *-10 Blazer in an '85 C-20 pickup and an '88 Olds Cutlass Ciera. And that same module would work in my '87 LeSabre, if I had pulled it before the Cutlass went to the scrapyard.