The MacPac kit includes a Y-cable adapter that is installed between
the mini's power button and the motherboard. This allows you continue
to use the original Mac mini power button as well as the P1900. The
MacPac and Y-cable allow you to easily remove the Mac mini from the car
so that you can take the mini inside for software tweaking or updates.
When indoors simply use the AC adapter that came with your mini.
The MacPac also provides access to the P1900 or P2140 secondary output, DelayOn amp control, and ground via seperate wires.http://carnetix.com/installation/IWill_power_cable_instructions.pdf
Please see the IMPORTANT installation instructions in the "Extended Information" tab.