Why not just run your tuning source through the HU?Is there such a thing? A DSP that has two or more distinct source inputs? Say one stereo input from the HU and another stereo input for tuning. My DSP is kind of buried and is a PITA every time I have to switch the RCA's for tuning.
What is your setup? What DSP? OEM head unit?
You should be able to accomplish this with a lot of DSPs.