My mom was talking about this movie... I didn't see it so my description may be off.

Something about a family out in their car.. it breaks down or something(?), they get out, wander off (I think it's winter), and find a cabin. They stay in it for a while and then the husband/father leaves to find help. He eventually goes back to the car, only to find it destroyed by fire, and all their bodies (including his) inside - they were all ghosts they just didn't know it.

Anyone?

No one? :(

Your description is a little off, but I'm pretty sure it's Voices (1973)
The whole movie is on YouTube - Voices 1973 1/9

Hmm, I don't think that's it.
My mom remembers seeing it when she was little, and she would have been 17 when that one came out.