As a last test, I took jumper wires with alligator clips and hooked the red and black wires directly to a battery. Nothing else attached to the deck. Just power and ground. No lights of any sort.
Repeat this same test but this time include the head-unit's yellew B+ as its the one that the head-unit actually draws power from. I bet the head unit will then power up.

When you used your test lamp to test the B+ on the red and yellow all the way to the head-unit, did you also test the B- (ground) circuit? Sounds like the head-unit B- got disconnected during the amp swap.