This is definitely a challenging issue to control. We don't have pest control expertise on staff, but can offer some guidance from similar past questions that may help. 

 1. Would ultrasonic devices work to keep them away?

Deterrents like ultrasonic sensors and scents are usually only periodically effective due to animals habituating to the devices/products over time.

2. Do squirrels hibernate? IOW do we still need to do something now, or should we wait until spring?

Not really, they remain active all winter.

3. Any other ideas? (Commercial pest control says they can trap them but removing squirrels one at a time is nonsense.)

In general, trapping and exclusion efforts combined are the most effective, but as you mention, when you're dealing with an entire population in a neighborhood, trapping might not be feasible. Excluding squirrels from vehicles parked outside is very difficult or impossible. As I mentioned, animals usually habituate to spraying scents or using noisemakers and such. However, they can be effective if you consistently refresh and change the type of product. There are dozens of DIY and commercially available rodent deterrents, and I don't have recommendations for any one in particular. Peppermint oil, cayenne pepper, and bitter apple are common ingredients in these sprays. Depending on the product, they can be sprayed on different parts of your vehicle (but usually excluding electrical or mechanical components). There's also Rodent-deterrent tape for vehicles that you can apparently wrap around car wiring. I have no experience with this, but would seem worth a shot given your issue with them chewing on wiring.

I hope this helps and best of luck!