Had the same issue on the '66 Coronet hardtop. I removed the kick panel, which allowed smooth removal of the interior door card after removing all the door bolt-ons like handles and armrest. Now that I had full access to the door guts, the latch adjustment was ever-so-slightly out of kilter, and I was able to unlatch the door by working the inner handle and pulling slightly on the mechanism. Came right open. Once the door was open, I made the adjustment necessary to allow the door to function properly. Plus, cleaning and re-lubing of all of the moving parts! Thirty minutes of work, total.