Check fuses 5 , 8, & 23. 5 & 8 are for radio; #23 is for amp. Fuse box under hood on drivers side.
Power into amp at # 1 & 9 at the amp harness (red and yellow wires), ignition must be on to measure power; should be above 10v.
If you have power going in, check the ground, with the power off resistance should be under 5 ohms at # 11 and 5 (brown wires). If you have power in and a good ground, replace the amp.

The power seats and the amp use the same ground. That is a good clue as to a possible problem. Replacing all speakers was a scam.

See post in this section for more info and wire diagrams: "2006 infinity Radio Amp"