Amp Cooling Fan Question

[chitowndad]

Ok, so I've got an after market amp in the front trunk and it got too hot today and shut down. I have a 12 volt DC cooling fan from Radio Shack and it has a red and blue wire. I hooked up the red wire to the amp power supply, which goes to the battery and the hooked up the blue wire to the amp switch wire from the radio.

Follow me so far?

When I connect the wires with the car off the amp turns on but the fan doesn't, which is ok except the amp shouldn't switch on.

If I connect the red wire to the batter and the blue wire to the ground the fan goes on, which makes sense to me

My question is why would connecting the cooling fan blue wire to the switch wire from the radio turn on the amp? It also makes a loud pop when I remove the switch wire.

What's the correct way to wire a cooling fan so it goes on when the radio / amp is on and off when the car is turned off?

Any help is appreciated.

Thanks,
-Steve