Found out it is possible to use cars for cover - they just have to be burned out cars. So, avoid any cars/trucks/buses/etc. with good paint jobs.
There are some safe cars with paint on them still, and some unsafe ones that look like they're safe based on the paint. The real thing to look for is the reactor, located where the trunk would be. If it looks intact, it's a live car; if it's missing, it's either dead or has already exploded. For trucks, it's the engine compartment, same deal. For buses, the only safe ones are the ones that aren't whole. When buses explode, they leave nothing behind, so it's only the ones that are placed as half-buses (like in the trailer) which you can enter, that are safe.
Motorcycles should be avoided too, but they don't explode as powerfully since they're gasoline-powered instead of atomic.