Here's the problem: When using the auto EQ feature, the HU shuts off its amp remote turn-on lead. So your bypass relay or device shuts off too.
You'll need to add a manual override switch to make it look like a real parking brake is connected. I'm fairly sure it would be safe to wire that in parallel with the bypass device (so your switch connects the HU's parking brake lead directly to ground). The switch can be a simple 2-terminal (SPST, on/off) type.