I just bought a 2003 BA xr6 n/a and am looking at interior mods that can be done.

probably the simplest and best mod you can do is get an aux cable from ebay and connect that to your fusebox

gives you aux right away !

also i changed my dome light bulb and got a smd led light from ebay ! shines BRIGHT WHITE looks amazing and is only like $5

and or you can have blue map and dome lights to suite to rest of the blue look ! turns out really nice...

if you havnt done it a set of HID head lights makes the car look amazing also

good luck

Depends how deep your pockets are as far what mods can be done to the interior. If you have the stock xr cloth seats you can change them for leather XR, Fairmont, Fairlane, GT or GTP seats generally without a problem. If you have the stock Blue LCD screen ICC you, can swap it out for a full colour screen, in Grey, Silver or Piano Black. You can change the dash face to any of the XR or GT range without much trouble. You could change the steering wheel, there is quite a selection of nice Ford leather wheels on Ebay, as well as shift knobs and gates and surrounds. As mentioned the LED interior lights are an easy mod. Get the windows Tinted etc... Generally if it was ever fitted to a BA or BF (and some FG interior Pieces seats etc..) then it should able to be fitted to the XR with little or no problem. Easiest way to see whats out there is just type BA BF into ebay and see what comes up.