[Imp]

As far as I know its the smoke generated by the wreck that gives cover so yes wrecks provide cover.
Tank question is a strange one units are never a terrain feature as covered when you asked about bunkers so dont provide cover. Also as already covered in one of your posts if the tank is destroyed they will likely take a big morale hit.
Then there is the obvious stuff
The unit that entered last spent an extra MP to do so, so easier to see & it sees worse & has possibly lost shots.
Tanks attract fire & a unit stacked with it is suscepible to splash fire as suppresion or even if unlucky being hit by a near miss, as a player thats one juicy target.
At range also lets me suppress the infantry unit I could not see by firing at the tank perhaps to the point where I can see him.
So tanks & infantry in same hex is a only if absolutly have to move