Well, it could be a battery problem.

There is almost certainly more IR radiation in a car environment than in the average home.

Therefore you may find with a weak/on its way out battery that the extra IR present in your car swamps the IR signal from the remote stopping it working like it does at home.

Replace the existing battery with a new one and see if that fixes the problem in your car. If not then we know its not the battery and you can begin to look at other things.

However ion that case a strong IR source in your car would be likely problem then, but I have no idea from where it would be coming.