I'd leave a pretty straightforward voice message or text saying you urgently need him to call you back within 24hrs so you can arrange collection of your property, otherwise you'll be engaging legal assistance.
A letter from a lawyer will cost you, but it might just prompt him to get in touch with you. He can't claim you've just sprung it on him if you've given him fair warning that you would pursue legal intervention if he didn't contact you.
Even though his health might might not be 100%, he still has a responsibility to you in regards to the car and at least needs to let you know what's going on in regards to your property and to make arrangements to collect it.
Goodluck buddy - it's a bit of a shit situation you've found yourself in, but I'm sure if you handle it properly, it'll work out for you.