as someone mentioned earlier, what about your headunit's AUX input?
if not that, some DSPs offer an AUX input as well. But you're still left with controlling that volume independent of the headunit.
So, in that sense, I would think you could just use another set of inputs on the DSP (since most nowadays have at least 6 channels of input) and then simply turn down the headunit volume and use your software/laptop to control the signal/volume of what's being fed in to that other set of inputs. I don't know this will work, but I think it might. The DSP doesn't know that it's not just a 'rear' channel of your headunit. This is assuming your DSP has multiple channels of input. In which case, just run a secondary set of RCAs to the glovebox and tap in to them when needed.