Within the FAQs we have here, on the OEM/Factory/Stock Audio Integration Products Companies post, I found that www.pac-audio.com makes a part model TR-4:
Senses voltages as low as 0.8 volts and gives you a 12 volt output with a one second delay. Trigger off of speaker leads or +5 volt remote wires.
• Add a remote turn on for aftermarket amplifiers in vehicles without a factory remote wire.
• Eliminates turn on pops in amplifiers
Or something like that, I'm sure If you search you'll be able to find some from a few different websites.