Well, I suppose the obvious is worth mentioning. If you have dealer installed accessories (e.g. roof lights, grilleguard lights) or a back-up camera, this is one possible issue. To go to "instant on" when you go into re3verse, the back-up camera has a low draw when the truck is off. The dealers have been mostly hooking their lights up to direct power (via relay, of course) and bypassing the IGN switched hot circuit. This means if you shut off your engine and lock your car, your lights will stay on until you shut off the switch. Mine are rewired to the IGN lead, so if the truck is off, so are the fog lights.
As I said, probably not your issue, but worth noting.
You can trace current, starting from the positive battery lead using a field ammeter. You don’t need to cut wires or even make electrical contact with the wiring. This meter measures the electromagnetic fields that always accompany current flow. If all else fails, a good auto electrical shop can trace the problem this way, if it is a current draw issue.
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