Every morning when I wake up, I have cat paw prints all over the car. Is there any way to harmlessly deter the cat(s)?
Cats usually dont like citric smells. If you can spray some lemon juice around the door or leave some lemon peelings that might help.
Funnily enough, you're not alone:
In case the link goes stale, the video linked above features...
....a homemade device with a motion sensor that activates a spray of water from the garden hose
Hilarity aside, it appears that there are a variety of low cost commercial devices that operate on the same principles as this jury-rigged version. The video author did note he'd tried a variety of methods(citrus peel, urine), even noting that the technique in this video wasn't necessarily successfully deterrent to the cat making an appearance (though it will probably keep it off th car)
I've never found much success in keeping the cat off the hood; he likes the warmth of the just-run engine. The answer will probably thus be quite mundane; cover the car or barrier it physically (garage), neither of which are truly lifehacks. For my part, I've accepted the paw prints as part of my life and cat's personality, and keep the area around the car clean to discourage dirty cat feet
You can provide a warmer place for the cat, like a heating pad, because the car will cool off during the night. The cat will prefer a comfy, constant source of heat instead of a hard metal car hood that becomes cold during the night.