A USB charger is only 5 volts, regulated down from usually 12V. Many of them work when fed 6V.

Before spending any money or much effort install or test a cigar lighter socket powered by your 6V with your charger/phone plugged in.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-