How to remove static

I just installed an amplifier in my car, and upgraded the speakers.  With the engine off, the sound is great--no problems.  However, with the engine turning over, there's this constant static.  If I pause the CD player, it's quite plain.  Note that it seems to remain the same regardless of the car's RPM, so I don't think it's alternator whine.  The shielded RCA's, speaker and power wires unforutentely are running rather close together (cramped c**kpit), but since there's no interference with the engine off I'm inclined to think that their proximity is not the issue. 

The car's body is galvanized, but I grounded the amp to the same ground point that the motorized passenger seat uses (8 guage, just like the +ve).

Any suggestions?  Is it possible that the ground is somehow insufficient?  Is there some 'noise' being fed into the power line from the alternator through the battery (power line goes straight there)?

Any hints on how to narrow down the problem?

Thanks in advance!

Does the static occur with  all sources ? ( Radio, CD or tape ) Check to see if the amp has a cold solder on the RCA and another thing is to check to see if the speaker wires are contacting each other in any way. Even if a strand is connecting across the terminal, you will get static. Make suer your ground wire is contacting the metal surfarce and if there's paint on the car or the seat make sure it is ground off with a sander.