'74 GTV brake light

[Loosej@domain.elided]

Alfisti,
  Today the brake light in my instrument cluster suddenly came on.  I first 
pulled the wire off the switch at the hand brake, the light stayed on.  Then 
i got under the hood and pulled all 4 plugs on the brake reservoir off and  
the light STILL stayed on.  The brakes work well and the reservoir is full.
  From the schematic, it seems that the culprit might be  device #13,   the 
"service brake pressure warning light switch" or a wire shorted to ground.  I 
don't know where the brake pressure warning switch is, all the wires from the 
reservoir go right  into the firewall... can anyone help me here?
Thanks,
Joel Lewis
'74 GTV
'89 Mialno

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