12volt power source from Mach 500 Stereo

Does anyone know if the factory MACH 500 stereo puts out a minimum 5 volt signal to turn-on/off some other device? I've checked everything that I can in the car at the back of the radio and I can only find one that sends out 4 volts (aux output). I'm trying to get my powered sub to turn on/off with the radio and I need a 5 volt source. I can use the accessories 12 volt source, but that means my sub will turn-on whenever the key switch is on, even if the radio is off.

Is there another location in the car where the radio switched 12volt power is located?

Any ideas or experience with this same issue?

Wow, I feel really stupid now! Yes, it's a SHAKER 500 system. I tried the AUX wire from the radio and it only had 4.6 volts, which wasn't enough to turn-on the amp for my sub. perhaps it's because the battery is at less than 12 volts, from sitting all winter long. I'll charge the battery and try it again. Thanks