I recommend a non-polarized capacitor over series connected aluminum electrolytics. The car audio people call these bass blockers see:

http://www.electronixwarehouse.com/car/accessories/bass-blockers.htm

Use a 100 to 150 Hz rolloff point, it is a one pole filter so the roll off is gradual

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!