Oh, my. I know it is tough, but you are going to have to step in and even take the car, in necessary. Both of your parents could be killed or they could kill someone else.
It sounds like they both need some type of care. Is your mom willing to move to assisted living?
We had the same problem with mom driving long after she was repeatedly asked not to. We had to take the car to our house for storage. Auto theft you say? Not really. Once the MD says you can't drive, your license is suspended and no Judge in his right mind will allow you to stay on the road.
I assume your folks are still living on their own. Driving gives your dad independence and it's very hard to take this away. He cannot get by with the mom as co pilot thing. You can read many posts here of that same scary stuff people have gone through with elders. Research the topic on this site. I'm in a similar situation with my 85 year old dad. His doc says he's ok to drive a little longer and he just had his license renewed for 5 more years....great.......But it's getting dicey and I'll probably end up in a physical confrontation with him when the time comes to get the keys. Take a look at things...Is the car scratched up? Is he getting lost? Take a ride with him. Did he scare you? If this stuff is happening do whatever it takes to stop the driving. Lie, fib, disable the car, have his doc write a letter, report him to the dmv as dangerous. (Most states will accept a report from anyone concerned and require a test. In my case, WV, they have the regs on paper but everyone tells me the state does not enforce it). If you are successful tour folks still need some way to get around. Start checking into local services before you pull the trigger. Don't screw around with this. Look at the situation and take appropriate measures.
It sounds like they both need some type of care. Is your mom willing to move to assisted living?