[JRTJH]

You should find an electric outlet box with a blank plate just forward of the roof vent cutout. Remove the cover from the outlet box and the 120VAC wiring (and possibly the thermostat wiring) will be inside that box. It is close enough to the 14" roof vent cutout that it will be covered by the air conditioner ceiling grid wen the air conditioner is installed...

Here's a photo of what to look for. The decal may/may not be present.
In the second photo, you can see the plate, just forward of (below) the roof vent location with the ceiling trim strip between them...

I found these photos on another website, they are not mine.