The light comes on when the handbrake switch grounds the circuit or when the differential pressure switch in the engine compartment grounds the circuit.

A wiring fault shunted to ground (abrased or simply a connector touching the body) can also cause the light to appear.

Is there anyone who has a speedometer from a 240Z and can send me a picture of the back of the speedo.  My latest issue on my 73 restoration is that the brake light warning light stays on all the time when the lights are on (only when the lights are on).  I'm thinking that on reassembly I may have mixed up the instrument bulb with the warning light bulb. The warning light stays lit when the lights are on even if I disconnect both the emergency brake switch and the brake master cylinder warning switch.  Since I can't see the back of the speedo, any switching I would have to do I'd have to do it by feel  A picture would help greatly..   Thanks!

What's happen when you pull the parking brake lever with the key on?  If a socket is swapped an instrument bulb should light up instead, and the warning light should stay off.  Might need to try it in the dark.

2 hours ago, z3beemer said:

Is there anyone who has a speedometer from a 240Z and can send me a picture of the back of the speedo.  My latest issue on my 73 restoration is that the brake light warning light stays on all the time when the lights are on (only when the lights are on).  I'm thinking that on reassembly I may have mixed up the instrument bulb with the warning light bulb. The warning light stays lit when the lights are on even if I disconnect both the emergency brake switch and the brake master cylinder warning switch.  Since I can't see the back of the speedo, any switching I would have to do I'd have to do it by feel  A picture would help greatly..   Thanks!

The gauge illumination and brake light sockets are next to each other. On the top, the high beam light and gauge illumination are next to each other. The brake light bulb has 2 wires, and the gauge illumination has 1 wire, grounding on the speedometer.

The dimmer should also cause a change if it's an instrument light.

Well, the brake light was definitely in the wrong hole.  I was able to correct that, and it now works as it should.  The high beam indicator also works correctly.  Now I have to try to find the open hole to insert the  instrument light (by feel).  It's times like this I wish I had smaller hands and less arthritis.