Every photographer needs to pay attention here because many of us will get gear stolen from our vehicles sooner or later.
Insurance will usually cover gear stolen from a vehicle but only under certain circumstances. Usually there must be signs of forced entry and some insurance companies require that the gear was stolen from a locked compartment (i.e. the gear wasn't left sitting on the front seat). You must also file a police report.
The cabin of a vehicle is not considered a locked compartment. This means that hatchbacks, minivans and SUVs usually do not have a locked compartment. But there are a few exceptions - I once had a Taurus wagon that had a locked compartment under the rear deck.
If your policy requires a locked compartment, and your car doesn't have a trunk, then you should buy or make a lockable box that fits in the back of your vehicle and is either bolted or chained to your vehicle.
Insurance deductibles are usually much higher for gear stolen from a vehicle.
If nothing else, go to a hardware store and buy a long, lockable steel cable (ideally plastic coated) that you can loop through all your straps and handles and then secure it to your rear seats. This should at least slow the thief down.