Both the police and the courts are likely to look at the situation as a whole, rather than adopting any policy specifically in relation to drivers or owners.
For example, it's unusual for people to rent cars to their friends for months on end. That might suggest there is something untoward about the arrangement.
Is there evidence of the commercial arrangement, or is the owner pretending to have rented the car out for months, when in reality they had stashed the drugs then let a friend borrow the car for a day?
It would also be unlikely for a drug dealer to stash a large amount of drugs in a car then lend the car out on a long-term basis, so if the car is out of the owner's hands, that would tend to suggest the drugs belong to the person in possession of the car (and not the owner).
But if the amount of drugs were small, typical of personal use, then it becomes more credible to imagine they could be forgotten by the car owner before lending the car to a friend.
Police intelligence might also have a bearing. Does one party or the other have known links to the drugs trade?
Also, is the lifestyle of one or the other, in particular, inconsistent with known sources of legitimate income?
My point with all these questions is to highlight how sensitive the issue is to the fine details of the circumstances, and that it's impossible to give a strictly general answer.