When I think of "briquettes", I think of small-ish, individual pieces of charcoal that come in a bag, which is normally used for grilling meat. I make no distinction between "briquettes" and "charcoal". I'm US btw.
That being said, you could measure the interior of the vehicle's interior length, width, and height, multiply them together, and get a decent approximation of the interior's volume. Once you have that number, there is a "guideline" that's used to calculate the MINIMUM amount of charcoal needed for the space you previously calculated. That guideline is 0.135kg/m³. Since it's the minimum, it would be a good idea to multiply whatever amount you calculate by 3, just to be sure you're using enough.
Charcoal is lit afire outside your chosen space and allowed to burn until all the charcoal is burning red-hot. It is at that point that the red-hot charcoals are moved into the space, the space is again closed off, and the charcoal is given some amount of time (20 minutes) to fill the space to a lethal level of CO. After the 20 minutes goes by, one enters the space, closes off the space quickly, and procedes to breath in the odorless and colorless CO gas.
In a vehicle you will need to be careful where you put the hot charcoal, so as not to start a fire. Bricks on the floor work well. It is best to use small metal pails to put the red-hot charcoals into. The best charcoal to use is probably lump-type that is 100% carbon with no additives.
If you have any other questions, feel free to ask.