It's both, as it turns out!
We bought this truck with a recent V8 installation and rewire, among other things. The engine would run 5-8 seconds after turning the key off. Another post on this forum indicated that the electric cooling fan could act as a generator and cause this issue.
The person who rewired the truck did a pretty good job, but had all switched circuits powered through one fuse and connected directly to the ignition switch. This had already caused the engine to lose ignition power when a turn signal wire shorted.
I added another fuse panel and powered it from the ignition switch. Now I have fuses for individual circuits. I also installed a relay for the cooling fan with power taken from the hot fuse panel and control power from the ignition switch. Now the engine shuts down as it should. I looked into simply adding a diode, but when you get above 10 amps they start to get expensive.
