Get a set of walkie talkies. We use phones when cell service is good, but usually have to use the walkie talkies. Also important to not use terms like left and right, but use positive directions such as driver or passenger. You also need to agree which end these directions apply. I keep telling DW that I want her to tell me which direct the back end needs to go.
The Long Long Honeymoon folks posted a video a few years ago about this. Its directed towards backing up a trailer, but the concepts are still the same
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