The one that’s flashes with the key is on is the + side of the CAN network. It won’t carry enough current to turn on the head unit.

How the original worked is battery constant power with a ground of course and the wire that flashes is the “network wake up signal” so the original will wake up when the network is active.

The easiest thing to do on this style is to tie into the keyed power at the HVAC controls directly beneath it.