That is exactly what I did.
I jumped the "D" coming out of the head unit (the switched lead with the ignition key 12 volts) to "K".
See the link below for the diagram:
http://www.carstereohelp.net/wireharness_Infiniti2.htm
Please note that I had to jump the connection "D" directly to "K" while "K" was still connected to the head unit & wiring harness. If "K" is connected directly to "D" without being connected to the head unit the amps will not turn on.
Also, "J" coming out of the head unit has 5 volts output. In the diagram above it is the ground so I just cut the wire in the harness & electric taped the ends.
Basically anytime the car is turned on the amps are also on. I did not want to go this route but also did not want to install a separate swith just to turn the amps on.
I think it is no different that having your radio on all the time.
Someone please correct me if I am wrong for doing this or if there is any danger in running this setup like this.
I have been running it like this for 1 - 1 1/2 weeks without any problems.