Repair-It

Do you think it's my kids?

Nah, it's definately alternator noise.

I tapped into the preamp outputs that went to the stock saab amp using a din connector and a 4-channel phono cable. Kept the stock HU and amped all four speakers.

Speakers are infinity kappa... 6.5" components in front and 6x9" in the back. Rockford fosgate amp to power them..

Should I try to ground the amp and HU to the same location?

Ah, phono cable. Your problem is that you're using a record player in your car. Or at all.

Hooked the amp back up and the humming is still there. If anything, it's worse.

Rises and falls with RPM. I guess I'll need to begin diagnosis by disconnecting the signal cable at the radio end and seeing if the hissing persists. If it stops, which is unlikely, then the problem is probably the radio.

If it doesn't, I can remove the signal cable at the amp end and determine if it's the amp or the cable. If it stops, which is likely, then the problem is in one of the soldering jobs that that cable required. Remember, one end is a DIN connector that I installed after removing the existing phono connectors. The other end had a failed phono connection, so I had to reconnect that as well.

If it doesn't, then I guess I have to clean up the power running into the amp somehow. This means making sure the ground is good. Actually, that sounds pretty easy. Maybe I'll start with that.

I recommend trying to bring everything in the car to the same ground plane. Meaning you may have to add grounds to the body. The head unit and amp should be equivalently grounded.

Try disconnecting the signal into the amp and see what that does. As you alluded, the answers to each diagnostic stage tell us something different.