How to connect multiple signal wires to a single output?
Probably the following is just what you expected:







simulate this circuit &ndash; Schematic created using CircuitLab

The brake switch will be attached as usual to the computer/whatever that does its normal stuff. But you will hook up a diode to that node/line so that you have separate access to the signal for the radio through \$D_1\$. But \$D_1\$ also keeps the computer from "seeing" your bypass switch. The radio's EBRAKE signal can see either switch condition, but the car's computer can only see the EBRAKE switch condition.

It is basically a "wire OR" arrangement using diodes. Very easy to understand and apply. You might be able to eliminate \$D_2\$. But I'd keep it in.

If the diodes drop too much voltage, you could consider using Schottky (or hot-carrier) diodes for a lower voltage drop across them. Or you could even go transistorized, I suppose. But the diodes will probably be fine.