Looked up the part#, even found some alternative part# but none of these feature it the TR7/8 parts catalogue........not that that would mean much!
I can tell you it is a 1amp diode and is used in many applications and different cars including the Deloren, which apparently has 6 of 'em the scatted though the car.
Here is a circuit diagram I found for a Land Rover where it is used for the rear window heater:
I think that this is an odd way to go about this. Maybe to shut this and other circuits down to conserve the battery when the volts drop below below a certain point.
But it is also used on some Jaguar hand brake light systems, where it is used as a blocking diode. It's job is to allow the low brake fluid warning switch to activate both the brake warning light and the handbrake warning light, but only allow the hand brake switch to activate the handbrake warning light.
I gather the reasoning behind this is to throw up an extra warning light in the hope that it might be more noticeable.