Review on Enhance Your Audio System with the PAC TR-4 Remote Turn-on Module by Eric Longoria

Very small, fits easily behind the dash, clear instructions, works as described, uses the speaker wire as a trigger. Just a note, the 6 volts DC present between the factory stereo speaker output and ground will remain on for approximately 30 seconds after the radio is turned off. It also turns back on when the doors are unlocked remotely. But it turns off again if you don't start the car. This has not caused any problems and appears to be normal for CAN-BUS systems in new vehicles.