Engine Noise through stereo

If it comes through the radio only (not the cd) it has to be coming in on the antenna and no inline power filter will have an affect unless the radio design is as such that the radio section is totally isolated inside from the CD section. Very few are designed like that. A high performance ignition such as a Mallory or MSD multi spark box can magnify interference since the spark energy is so much higher and many more times per cycle than a stock ignition. If you have solid core or very low resistance plug wires that's another strike against you.

Resistor spark plugs do very little to stop radio interference in spite of being advertised as such since the beginning of time. Spark plug wires have a much greater affect on RFI (radio frequency interference).

The antenna usually doesn't cause a problem unless the coax cable is super cheap. The proper length for a FM antenna shaft at 98 MHz (center of the band) is 29". If it's not somewhere between 26 and 32.5" it may not be resonant on the FM band which can allow greater interference.

A side note: To optimize your radio listening follow this rule of thumb. If you mostly listen to stations lower than 98 MHz use a whip in the 29-32.5" range. If you listen mostly to stations above 98 MHz use a whip in the range of 26-29" length.

One last thing to check is to make sure the ground strap from the engine block to the firewall is in good shape. If not it's like having a spark generator under the hood. When the engine is grounded good to the fire wall it can help kill off the spark generator affect.