All winter, my 1985 Suburban had no problem starting back up after running and getting up to temperature. Last summer, if it sat for ½ hour or so, the glow plug light wouldn't come on, as the engine was still too warm. Yesterday I got a reminder that winter is nearly over. A warm day, the engine was warm, and it sat for a little over ½ of an hour, and the glow plug light didn't light up. I'm sure the compression is not what it once was, so that's the likely cause for it not wanting to start up... Today I read a lot online, and decided it's a symptom of the glow plug inhibitor switch, a normally closed switch... Eventually I found the one on my engine, and looking at the wiring diagram, came to the conclusion that if I unplug the harness from the switch, the glow plug light on the dash won't light. So, after unplugging it, I turned the key, and find that the glow plug light still lights up. Any holes in my logic that some kind soul will point out to me?

I took a picture of what I think is the correct switch, on the passenger side, near the firewall...