Hmmm. I think it works like this. The steering diodes in the instrument cluster are used to isolate the other idiot lights from each other and the battery lamp, yet allow the lamp test feature to work when power is applied and the small red lead is something less than 12V, near ground at engine off.

When the red lead goes low (not open) but low like when it shorts to the clamp under the damper, Yogi says it is just like lamp test all over again. All the idiot lights pass current through their isolation diodes to the ground out there by the alternator. If the diodes weren't there, then the oil pressure sender would light all of them when you lost oil pressure (probably a good feature for at least one of my family).

I suppose if you have experience keeping a British car electrically sound, you can abide the Bosch/Hella/Cibie/VDO stuff.
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Art Benstein near Baltimore