Here's a weird one that isn't intuitively obvious from the wiring diagrams. I have a '99 Mountaineer that I picked up recently that included some known problems. One problem it had was the power windows not working. Another seemingly unrelated problem was the following behavior:
Instrument panel dimmer switch ON, only driver's door open => dome lights and overhead display cycle on and off (sounds like the turn signal).
Instrument panel dimmer switch OFF, only driver's door open => dome light and overhead display do not cycle.
Instrument panel dimmer switch ON, driver's door open and any other door open (including tailgate) => dome light and overhead display do not cycle.
Instrument panel dimmer switch ON, any other door open by itself (including tailgate) except driver's door => dome light and overhead display do not cycle.

The ground wire for the power windows failed between the driver's door and the body. Nothing in the troubleshooting procedures included the above behavior in the symptom charts. Repaired the wire and the windows worked and the cycling went away. Strange. The only thing I can think of is this is somehow related to the GEM as the power windows, dome lights and door switches appear to have inputs to the GEM as well as the GEM having connections to a number of relays that are involved. I think this illustrates the problem with electronics boxes. They are shown in the wiring diagrams as only a box with nothing inside - no logic diagrams, no circuits, etc. shown inside the box - and only connections to the box are shown. Every once in a while, there will be a little text box with a very brief description that provides a tiny bit of logic, but by and large, that information is hidden from the mechanic (on purpose, it seems). Other industries I've worked in provide the logic in their control systems, but not automotive. Frustrating.