If I were malicious enough to design the system, I would make it a heartbeat. Skip too many heartbeats and your car bricks. It could be written in to the terms of the loan since companies are using in-car computers for repossession.

“Why is my car bricked?” “Because you tried to disable our payment verification system.” “I live in a rural area.” “You’re like 1% of customers. Your loan contact says you have to drive within cell range once a month. Fuck you, we’re repossessing the car and keeping the money anyway.”