Emma,
The simple response to your OP is as follows:
The payment you receive is net so tax has been deducted by your employer. The gross amount is ...whatever (someone else can do the Math) so in the same way that you report to your Lender your gross salary, you should report to them the gross amount of this benefit you receive. They will want this evidenced in the form of a salary cert, which should include benefits such as this, from your employer.
One doesn't need to actually own a car if receiving "car allowance" payment and its a taxable benefit given by many employers.
Thanks,
Jim