Crossover for my Tri-Mode amp setup

Hi all

I'm putting in a 12'sub and want to wire up in tri-mode, my sub and my front splits ( for now anyway.....may do a seperate amp when the budget allows later on....) and my question is - if I run my Rockford Fosgate amp in full range mode to allow the splits to operate properly, I need to put some sort of crossover / capacitor on the subs speaker wire

I had this years ago in another car and it worked fine. The thing the stereo guys did back then was wired in a thing onto the positive side of the sub from memory a little cylinder type thing that looks a 'c' or 'd' shaped battery and then inside the sub box was a 2inch diameter coiled wire.....

Any suggestions what farad or microfarad I would need for a 12' 200W RMS sub at 4 ohms with a punch 40 amp supplying around 100W RMS to it.....

cheers