Now that I have the Faceplate Adapter Kit. Will a Rockford Fosgate 8 inch subwoofer fit in the rear deck of a 03 XJR? Need help and suggestions.

 

In a word no, it's a 6x9 hole for a start, the best thing to do is look for a shallow mount sub. Or, make a spacer to fit the one you want, you'll have to alter the cover for the parcel shelf, delete the grill, cut the new speaker shape out and mount it all so it looks reasonable. The petrol tank sits directly underneath the factory sub, with virtually no space between the two.

 

Only if you make a spacer so it will be mounted above the parcel shelf, instead of below. If the magnet and bottom of frame is small enough, they can fit through the existing opening, or the sheet metal can be bent downward somewhat to afford some extra room - how far into the hole the frame will fit will determine the overall height of the spacer and sub above the parcel shelf.

The other issue - is the Rockford Fosgate you are hoping to use a dual voice coil speaker? If not, then you lose half the power provided to the sub by the dual amp channels in the Jag (and, if all or most bass happens to be in one side of the stereo signal in a particular selection, you have a 50/50 chance of missing it). OR, are you even using the Jag amps anymore, with your new head unit?

I have several 8" dual voice coil subs lying around and have considered trying this for a while - it's just not high enough on my priority list yet. I think a decent, conical spacer could be fabricated that would not look too out of place, especially if painted in krinkle black or perhaps covered with leather.

The opening into the trunk will restrict air flow slightly, but I believe it is large enough to allow for good compliance and shouldn't reduce the sub's performance much, if at all.