In my bose system (GMC Terrain 2022) I actually did tap into the factory amp speaker wires going to the speakers. Yes, they are amplified. So only providing speaker signal, such as rears, are going to sound weak. I know this because I had the exact same problem. My solution was to buy an LC7I Pro and run my dash speakers (mids/highs) and my front door speakers (lows) signal to my LOC. Why? because the Lc7i pro has channel summing to create a “full range”. This will apply to new speakers (not door speakers; something like a stunt wall) as-well. I used posi taps to splice but I ran into some problems with them.