Look behind any air conditioner compressor or heat exchanger and you'll find at least one large hole for plumbing and wiring. It's usually bigger than it needs to be and sometimes only partially plugged with a large wad of electrician's putty, making it a good spot for heat to get out and mice to get in.
Make these holes more weathertight and less unsightly with a 1-1/2-in. rubber plumbing boot (available for about $4 in the roofing section in home centers).
Gently work out the putty (Photo 1) to avoid damaging the pipes or wiring. Replace torn or missing pipe insulation.
Using scissors or a razor knife, cut the roof boot to fit around the hole and to match the width of the piece of siding (Photo 2). Cut a slit in the bottom edge so the plumbing boot will fit around the pipes.
Spread silicone on all four sides of the hole and at the slit at the bottom to completely seal the edges (Photo 3). You may need to tape the rubber to the siding temporarily until the caulk dries.
Let the caulk dry overnight, then partially fill the boot with foam; seal the hole from the inside as well. Use minimal-expanding foam to avoid opening the slit in the rubber boot, and push the tip several inches in so the foam expands into the hole instead of out the top of the boot (Photo 4).