Alan,
From WSM 124, OD section, it appears there was no warning light on SI, SII. But it shows the SIV did have a warning light. Probably SIII too. Interestingly the wiring to the warning lamp ran thru the light switch or panel lamp switch. You can see this also in the SIV wiring diagram.
WSM 145, OD section, shows no warning lamp, but I am 90% sure the wiring harness included provision for it- at least the harness I got from Jeff Howarth included it.
I am 95% sure that the location for OD warning lamp is above / left of the Tach. If you look (or feel) at the rear of your dash in that area you will find a molded raised area similar to the molded area for the High Beam and Turn Indicator lamps. I think the hole is NOT molded thru and in my case I drilled it through, initially from the rear with a small drill and then enlarged it and installed a spare Ignition Warning light. I used a faded Ign warning light that is less red than the Ign warning light.
Today I would recommend adding just an LED lamp and resistor (or LED lamp with resistor built in) and wire it to term C1 on the relay, where the yel/purple wire connects. The LED will take virtually no power from the solenoid and yet, when lit tells you that the solenoid is getting 12 V power. The original warning light circuit really only tells you that the OD Relay is ON. But a common problem is that the relay is ON, but the contact for the solenoid is not made, so you have the amp lit but no power to the solenoid and you start thinking you have a serious problem inside the OD box, when you only have a bad relay contact.
By the way, for those interested, the diagram in the OD section of WSM 124, is the only one I have ever seen that shows a unique electrical aspect of the OD solenoid. It shows the two coils - High current Pull-in coil and Lower current Hold coil and the internal switch that opens the High Current circuit after the Pull-in is completed. I just discovered that dwg while researching this reply and have wondered about it for years.
Tom
Tom