The only workaround I have heard of for these systems are modules you need to splice into the wiring harness like this one. They are fairly straightforward to install, and if you aren't that handy yourself, your local pro-car-audio install shop should be able to hook you up with one "to keep your passenger quiet".
Answers & Comments
Firstly just try it with the park-brake disconnected as some run open loop = ok
If that does not work it will be closed loop = ok - that means just ground the wire (attach to the body of the car to achieve 12v-neg)
It will be one or the other but do it in the order above
Best regards
Lloyd
The only workaround I have heard of for these systems are modules you need to splice into the wiring harness like this one. They are fairly straightforward to install, and if you aren't that handy yourself, your local pro-car-audio install shop should be able to hook you up with one "to keep your passenger quiet".
I hope that helps.