If the OD is engaging and disengaging without a change in the light, I would begin to suspect a low fluid level. Low fluid level can cause delay in engagement. Fluid level may be easier to check than the wiring. If you've had even a slow leak from the speedo cable connection or the upper two OD nuts which may gather at the OD solenoid, leaving a trail of reddish Type F on your floor, I'd definitely check the level first. If the fluid is OK, then I'd try to isolate the electrical components.

Good luck with it!