This is kind of a complex situation (chase, vehicle damage, character damage, thrown weapon, oh my!) so let's recap step by step, starting with turn order. I'll assume a vehicle chase but the situation would be even simpler in a regular, grid-based combat.
Note: I'll mention some page numbers, they're for the English version of the book.
- The driver makes a Driving roll, getting an Action card as per usual chase rules. Deal with complications as needed.
- If the driver does not have advantage (ie. lower Action card than opponent) no attack is possible this turn.
- Otherwise, everyone aboard the vehicle gets the same initiative. We'll obviously assume the PCs can act.
- A PC throws a grenade, making a Throw roll as usual: apply any bonus or penalty due to speed, terrain, range, etc. (see Attack Modifiers under Vehicular Attacks): you'll find that throwing grenades in high speed chases is seriously risky business! Keep in mind that grenades use the Medium Burst Template.
- If the roll succeeds, the grenade hits the vehicle and goes off (yay!), the vehicle will be affected. Check if any other vehicle gets affected by the blast (might happen if multiple vehicles are very close to each other).
- If it fails, things get trickier because deviation rules apply. Apply the following for all vehicles involved.
- Thrown weapons deviation is 1d6" times 1 for Short, 2 for Medium or 3 for Long range. The direction might be important, too: your own vehicle might get caught in the blast!
- Grenades use the Medium Burst template, which means a 4" diameter. If the vehicle is more than 2" away from the impact (we want the radius here, not the diameter), it's unaffected by the blast.
Let's suppose the blast did hit at least one vehicle. How does it affect it and its passnegers?
- For the vehicle: regular vehicle damage rules apply (p. 100). To sum up:
- Roll for the grenade's damage.
- Damage under the vehicle's Toughness gets ignored.
- Damage equal to or over Toughness: driver makes a Driving roll or goes Out of Control (see table p. 101)
- Raises cause a Wound to the vehicle and a Critical Hit to be made (see table p. 101).
- For passengers:
- Affected passengers have restricted options : they can't avoid the attack unless they jump out of the vehicle (which may or may not be an option!)
- The vehicle will act as an obstacle for the attack: see the Obstacles section in the Situational Combat rules (p. 75). The GM uses his best judgment as for the bonus to apply here: in a regular car, for instance, the lower half of your body would be covered by the door's metal while the upper half would only have a regular window in the way.
- Add this value to the passenger's Toughness, roll for the grenade's damage and apply the regular damage rules here for Shaken status and Wounds.
I think that's it. Let me know if I missed something or if you have further questions.