I guess what you are trying to do is use the handbrake as a safety device, so you cant "POP" the boot unless the handbrake is ON.
If I understand correctly your problem is when you connect to the handbrake switch, the popper still operates, even when the handbrake is OFF.
How have you got the wiring configured for the "popper"?
When the handbrake is OFF the feed to the handbrake switch is ON (12V), and when the handbrake is ON, the feed to the handbrake switch is OFF (0V) (The handbrake switch grounds one side of the brake warning lamp, making it illuminate).
See enclosed drawing (note the top drawing shows the way the handbrake switch works and is drawn with the handbrake ON)
Therefore your "popper" circuit must use the handbrake switch as a "Ground Path", so your circuit will be something like:-
12V to the popper switch.
Popper switch to popper device.
Popper device to handbrake switch
See enclosed drawing
The diode is optional, but note where it is positioned!
I would try without it first as should not be necessary.
I would also advise taking the 12V for the popper from an ignition feed, as if you take it from a permanent 12V then the diode
will be required.
A schotkey diode is recommended, as a standard diode might cause the lamp to dim slightly.
The diode only needs to have a current rating of about 500mA to 1 Amp, as it only needs to pass enough current for the bulb.
Peter