You have to enter the number of KM's you drove in total and the number of KM's you drove for business use on the T2125 - Statement of Business or Professional Activities form, under Chart A - Motor Vehicle Expenses. The percentage of your total expenses you can use for business is the business KM's divided by the total KM's. The vehicle expenses won't calculate until you fill in the KM information.
Technically, you need a mileage log to prove your business KM's travelled. Whether CRA will ever ask to see that log remains to be seen as they select different people for audit depending on any red flags that are raised in the data you are submitting. If they feel that some of your expenses are not "reasonable" for the type of business you are in, then you can almost certainly expect an audit and will have to produce a mileage log. If you don't have one or can't reasonably piece one together, all your vehicle expenses may be disallowed.