12V source indicating air parking brake (push-pull) engaged?

[TheHubbardBus]

Bus is 2003 IC 3800 with air brakes.
Our stereo receiver requires a 12V signal from the parking brake switch to enable certain fuctionality (for example, you have to have the parking brake engaged to set up the rear-view camera's guide lines).
There's no pressure switch or any electrical connection to/from the push-pull valve itself.
The gauge panel, however, indicates the brake is engaged, so it's getting this signal from somewhere.
Does anyone know where? I don't know which of the countless wires connected to the guage panel is responsible for this signal, so I can't even track it back from there.
I know I could make this work by other means, but I'd rather hook it up exactly as designed.

[s2mikon]

Some buses had a brake light switch just for that. Mine was at the rear axle. When the ignition was on and air pressure was released from the spring brake system it would turn on the brake lights. I don't think but not sure it was a DOT requirement but set by local districts. If you don't have one you could use a hobbs switch to complete the circuit when air pressure goes off to zero. These were 2 terminal switches used for oil pressure lights and alarms.

[TheHubbardBus]

Thanks, s2mikon. I think that's enough info for me to figure it out from here.