Still working through my install, almost there!

2013 Silverado Ext Cab, Non Bose

Where can I get a "harness" or "breakout" kit that fits between the existing harness and the stock head? Such a device would be "straight thru" for most wires, but break out the speaker stuff.

PAC does not this device. A few other (unnamed) online sources also do not have it.

Where else can I look? Does this thing exist? Barring that, I can make one...

(This is to eliminate cutting and splicing under my dash.)