The suppressor is essentially just a small capacitor, it works by ironing-out the waveform from things like the alternator (because the 14V is a pulsing signal as the commutator goes round). - If you think of a classic sine wave, you've probably got more of a saw-tooth pattern. A capacitor smooths off the edge of that to give you a smooth wave again. (Any electricians are free to correct me - it's years since I sat in electrical engineering 112)
It may not be the alternator, but that's a logical place to start because it generates electricity and will spin faster with the engine.
By electrical components, I mean the engine electrics; alternator, distributor, even the spark plugs or starter motor (but that shouldn't be spinning when you're driving).
Quite a few audiophiles will have a capacitor (as high as 1 farad) across their battery just to keep the power-draw nice and smooth... it's the strangest thing because the stereo will be quiet until the cap has built charge, and then it suddenly kicks in.
I would have thought that you could just put one across the amp if it's just those speakers that are picking it up... I would have thought it would be all of them.
Anyone know if a ferrite lug would help? That's not uncommon on data cables, but I don't think I've ever seen it on speakers.
A ferrite lug on the amp power lead might just take enough of the noise out (electrical noise I mean).