Audio Systems: bought used car and stereo has odd crackle, alpine stereo, 12volts

Question
hello, i just bought a used 1989 Saab and it has a pretty new alpine stereo installed.  it works great until i turn on the motor.  before the motor is turned on i can blast music as loud as the stereo can go. but after i turn on the motor it can barely reach loud enough to hear over driving without breaking up or crackling.

Answer
This almost certainly a fault in the power supply or earth, I would pop the unit out but leave it connected, connect a multimeter, set to 20 Volts DC, to the earth and power leads and see what variation accures from engine off, ignition on but not started, engine cranking and engine started.
It may show up a fault in the battery, alternator or battery connectors, earth lead, etc.
Should read about 12volts at rest, drop to 10v to 11v when on and drop to 9v to 10v when starting but rise rapidly to 13 volts with the engine on fast tick over and 13.8 to 14 volts when revved a bit.
Peter