I've got one of those in my 240. Sounds pretty good. Have the 9 pin plugs (a male and female) been cut off the wires? Normally you unplug the radio at its 9 pin plug, plug the power amp into the car side, and the radio then plugs into the male plug on the power amp. Going from memory. The big yellow wire is unswitched power. Go to the fuse box with that. The power on signal comes from the radio in the din wire. The big black wire goes to chassis ground. The small black wire If I remember right goes to the radio chasis. There is usually a lug on it for attaching it to the screw on the back of the radio. This is because the 9 pin plug for the radio on the power amp harness does not have the ground wire. I am guessing this is because the stock radios usually have the speaker grounds spliced into the main ground. Most power amps are damaged by this so the amp harness bypasses the radio ground wire. This would not apply with a 2 channel rear amp, but that amp was usually used with a 2 by 25 front speaker amp. I bet the guy that sold that to you sold the other one separately. So the little black wire replaces the stock ground wire. You guessed right on the speaker wires.