I finished adding 220uF capacitors to my Crossfire yesterday. I ended up soldering the caps into the wiring harness and using shrink tubing to insulate the leads and wiring. I also wrapped the whole assembly with packing foam and electrical tape in order to eliminate any possiblity of the capacitors rattling inside the doors. The Crossfire crossover conversion definitely reduced the bass coming from the doors and now I can finally fully appreciate the subwoofers. I still have to fiddle around with the bass setting to get it just right, but I am glad that the boominess is gone.

Thanks for the excellent instructions and the courage to go ahead with the modification.