Well on mine the switch that turns the light on to the trunk is connected to the ground side . So the wire to the light is always hot and when the switch is grounded it grounds the light and viola the light comes on .
Now since your loosing voltage somewhere in the line if you re energize it by using the next best wire in the trunk it would be your subs power wire you will have the steady voltage .
I'm not sure if the switch is on the ground on the 04 plus or the power .
Not so ghetto as you thought? One wire that's easily hidden...
