mach 460, h-u replaced - pop when OFF

I just replaced the MACH 460 factory head unit in my 1995 GT with a new Pioneer. I used the METRA harness 70-5510 that is a direct fit into Ford's harness (it even has the RCA wires to plug directly at the back of the new head unit).

I read a lot about installing a new H-U and many people reported a "POP" sound when the system is turned ON. I never read anything about having this "POP" sound when the key switch is turned to "OFF", and this is what I have...
I get no POP when I turn it ON, only when I turn it OFF...

I tried the 1K resistor on the AMP turn on wire since I read it could prevent the POP when turning the radio ON, but it did'nt solve my problem...

Anyone got an idea of what is causing this?

Thanks.

Yes, even if I turn the volume all the way down, it still pops.
The rest of the system is the Mach 460 stuff, front and rear amps, speakers...
The only new thing is the head unit.

Thanks for the reply.