That wire puts out 7volts
With a proper brake light, that will be pulled to ground, that will tell the amp the clutch switch is closed
If the clutch switch is open or the brake light bulbs are open the 7volts will tell the amp ‘no cruise’
If you are in cruise, ie the brake lights are pulling the 7volts to ground
And you step on the brake pedal, that battery voltage to the amp will say no cruise just the same
Therefore, yes you should not see batt voltage to that pin unless the brake switch closes
Someone messed your wiring up
89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.