It's impossible to throw a grenade to any vehicle. For example: to the hull (i dont know if it's right word, i mean back place of Zamak with seats) of Zamak or Zamak Dump where people can sit.
UPDATED: With today update by Bohemia we can see same problem with Ifrit's door. However, thank you for new patch, because it's looks like awesome anyway :) {F20251}
Im pretty sure none of the vehicles can have anything thrown inside them currently. Helis aswell as all ground vehicles. Id like to have this ability aswell though.
I agree I would love to ambush a truck and before they react frag the back of a vehicle transport or some such, hence why I think animated doors and hatches,etc should be implemented.
Up voted. I wouldn't mind an action command to drop a grenade inside a vehicle when able (like when the vehicle is unlocked and you want to blow it up but are our of charges).
Openable doors are being temporarily removed (until post-release).
Closing, since this is no longer relevant.