Looking at this with some stereo installation experience, Crutchfield is wanting you to tie the ANTENNA POWER (blue) wire to the blue/white remote wire on the 12-5vdc converter. This will be your trigger to turn on the subs using the 5vdc. The blue/red wire is the 5vdc trigger that is then tied to both of the blue/white wires which triggers on the sub amplifers using the 5vdc. The black wire on the 12-5vdc converter goes to chassis ground.The Ford trigger for the shaker subs are 5vdc trigger apparently or this step down converter would not be needed. That is the only reason for it.
Every aftermarket system I have ever installed uses 12vdc trigger top turn on the amplifiers. This trigger wire off of 95% of receivers in the aftermarket world is called ANTENNA POWER, because back in the crazy days of early stereo installation guys only had power boosters which turned in separate. The ANTENNA POWER was to operate a retractable power antenna if the car had one. Doing a full aftermarket system with amps you would not need this 12-5vdc converter. You would simply tie the ANTENNA POWER wire from the receiver to the REMOTE wire on the amplifier and this would turn the amplifier on when the radio was turned on.
I support soldering and heat shrinking the connections as well.
Call Crutchfield in the morning to verify my directions but I am pretty confident this is what you need to do. Crutchfield is awesome with tech support and service. I used to use them for tech support and they would help even though I may not have bought the unit I was installing from them. Great people all around.