The USB will continue to work until you crank the truck. At that point, the display goes to 000000 and the USB stops playing. You can depress the button that changes to radio and the radio works fine but the USB will not work again until you shut the truck off and then switch it on. With just the ignition switched on, it works perfectly again but as soon as you crank it, the same as above. I know it has to be the HEI distributor because with the points distributor it worked perfectly. I was an electronic technician in the Air Force many years ago but have not run into this before. It seems to be a proximity to the HEI problem since wiring it up next to the battery it works fine even with truck running. I also tried wiring the unit with both the ignition input and the battery input hooked to the battery wire (with the unit in the dash) and with that hookup you can play the unit without ignition switched on and the USB works fine. As soon as you crank the truck...USB acts up again.
