If you only had 3 volts then there is a bigger problem that needs to be addressed before you continue to hack away at your car. Instead of just jumping another wire from another source with unknown fuse ratings (or worse, none at all) you need to resolve WHY you only seeing 3 volts at the HU harness. Fix that, plug in your harness and it will work fine. Why risk damaging your car doing things the wrong way?
And yes, there is a memory wire that was clearly labeled in your instruction manual.