If you're sure everything is well caulked outside, including all along the bottom of the lowest piece of siding, and you think that this wall section is uninsulated, the easiest thing to do would be to cut out a rectangular piece of sheetrock at the top of the wall and pour some vermiculite into the cavity. When it reaches the height of the cutout, you can put a small piece of fiberglass insulation into the top, and then repair the sheetrock. It would be cheap and easy; but, if you don't have those skills, you're going to have to pay bigger bucks for a handyman...if you can find a competent one to do such a small job.

I'd remove the switchplate and use spray foam around the electrical box, assuming you have access.

Thank you kudzu9. But I'm confused about the vermiculite suggestion. When I google for it, all I see is horticultural vermiculite. Is that the same thing?

Thank you Sigrid. What type or brand of foam do you recommend? Not that awful Great Stuff I hope?

Your right that Great Stuff is awful! Two superior brands of foam are Hilti and Touch and Seal but you will have to purchase a spray foam gun to use them. Hilti makes a very nice gun but its expensive. You can find cheaper guns that can use the Hilti or Touch and Seal cannisters.