I noticed that this red plug was disconnected under the dash near the fuse box. I am not sure if it was disconnected by mistake when I changed out the fuses a few weeks ago. Not sure where it goes or what it does?

I only get power on the headlights and turn signals. I was getting power on 2 windows until I removed the fuse and put it back in. The fuel tank gauge doesnt work either. I tried sanding down the terminals a little because of rust but no luck. Thanks.

68 Cutlass Supreme

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That wire looks like pink?
If so it plugs into the ignition terminal on the fuse block. It goes to the power window relay behind the left kick panel. When it is energized it trips the relay which feeds the power to the window circuit. It probably came out when you were changing the fuse like you said.
The other problems I don't know? Search here for fuel gauge problems in old posts or maybe somebody will chime in. The chassis service manual has a diagnosis section for the fuel gauge, how to replace it, wiring.

 

Thanks for clarifying that. When you say the ignition terminal, do you mean the terminal on the right of the first fuse labeled ACCSY?

 

The IGN terminal should be the hole between the words HAZ and DIR SIG. However it looks like there is no terminal there. Is it possible to post a clearer picture of the fuse block?

The upper 2 teminals are hot all the time, so plugging the relay into one of those would work, but your battery would drain faster as the relay would be energized all the time.
In fact, something should be plugged into one of those top ports anyway....... Any other plugs hanging? Perhaps some in orange? Of course I am thinking about 1972..... 68 might be different.

Check the ground wire from the gas tank sender - they are notorious for bad connections due to rust and corrosion.

 

Sorry for the poor image. I took it quickly with my cell phone. This is a photo of the fuse box from the 68 assembly manual. Its a bit clearer to see. Might be different from your setup because I don't see a IGN input.

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From I what I see it plugs into the fuse box. Where it says accsy in between the wiper fuse and power relay fuse.

 

Given that last picture from the manual, I think your above idea is correct. Given that fuse is just 20A, it cannot directly power the motors in the windows - it must power the relay only. If it is switched 12V (check with volt meter ot test light) and the plug fits, use it!
The 72 fuse block is configured a little differently.