Since most people on this forum carry a blanket in your car kit, I suggest spray painting a large "SOS" on it in a bright contrasting color to use as a signal.

Spare blankets, tarps and even space blankets have a second use in sheltermaking. A secondary roof in hot climes will greatly help to reflect and reduce heat from the sun. In all climes a second layer will also create a still air pocket with increased insulation. Whatever is on top, unless your Snoopy crossing Nomansland make it garish and obvious.

A couple of those aluminum colored windshield sunblocking gizmos can work wonders too, and cost little. And if one has "call police" on one side, as some do, that could do great dual duty if placed on the top of your stranded car...

I recall a lady trapped for sometime off the road and obscured by brush in her wrecked vehicle. She used one as a solar condensor and secured water during the day.