On most maps I've seen so far, cars are a pretty common form of cover. However, they seem to have a tendency to explode, killing my soldiers. Some of them it's pretty obvious why they exploded, as they were on fire at the time, but twice I've had cars explode for no discernible reason, both times losing me soldiers and once causing me to fail a mission. Why are they exploding, and is there any prior indication that they are going to do so?
Cars indeed have a damage threshold, 2 of them, even!
First threshold prevents it from catching fire or exploding. (Pistol hitting a car for example) Then once that one is passed the car catches fire. The second one is where it explodes straight away.
Things to remember when using cars for cover or shooting at them:
From what I read, others report that grenades kill cars instantly, where in my games they never do that. It seems higher difficulty is higher threshold. I play impossible so as such I will not note my damage expectations and experiences.
The first indication would be the cars taking fire from either you or the enemy. You should see the car degrade from this, even if it's just windows shattering. And yes, I believe I have seen a car catch fire after it's windows were shot out. One assumes that in the future all materials will be flammable or explosive! In any case, so far I've observed a one turn delay every time a car that was on fire exploded, so there is time for soldiers taking cover to get out of the way in these cases.
It's possible to blow a car up without having it catch on fire first. The easiest way to see this would be to have someone throw a grenade at a car, or a heavy shoot a rocket at one. There's no delay there, it just goes boom.
One is given reason to believe that there is simply a damage threshold at work. At this much damage (call it X), the car starts to burn and will soon explode. At that much damage (Y) it explodes straight away. It's almost certainly true that enemy weapons (plasma) are more powerful than the initial x-com armaments, so they're more likely to explode a car as part of a volley than machine guns and assault rifles.
A few observations:
Anecdotally, I think that backs up the damage threshold idea.