Wrap a piece of chicken wire or similar, with smallish holes no bigger than 1/2", around the A/C unit, specifically on the surfaces where they gain entry. I've used short pieces of picture hanger wire like twist ties to fasten it in place. It's a great deterrent for unwanted entry into a variety of things, chimneys, soffit vents, any small opening that squirrels and chipmunks frequent. It allows air flow, too.
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)