The part you are describing from the Rickenbacker 4003 (c. 2005 or later) can be adapted to any passive harness, but to use it effectively you need to know how it works and what it's good for.
The part is a DPDT switch joined to a 500 kohm mini-pot. The pot functions like any pot would in that position--as tone control. The DPDT switch either makes or breaks a bypass circuit around a 0.0047 uF cap in-line with the bridge pick, AKA a high-pass filter.
This is useful in several circumstances. However it sounds really "thin" if you don't plug into a high input impedance when you use it. (500 kohms or higher.) The first and most obvious use is to modulate the tone of the treble pickup, by filtering out some of the powerful low frequencies. It gives the pickup a very high-clarity, airy sound. Now, the second and probably better use of it is to do this WHILE BLENDING PICKUPS, so that the two pickups are no longer in the same phase--the bridge pickup is in a perpendicular phase after the cap is put in-line. This means your mono signal will now have full representation of both pickups (it sounds HUGE compared to the other blend setting), and you get a cool mix of booming lows from the neck pickup and airy highs from the bridge pickup making for quite an awesome sound.
Sometimes, I ran my rig so loud at a show (these were rock shows usually) that I would just solo the treble pickup with the cap in-line. Due to the high volume and high input impedance of the preamp this did not sound thin, it sounded like a mega-articulate super bass. Basses, or at least my Rics, seem to generate plenty of fundamental even with that cap engaged (FT analysis proves me right on this). The sound can be a blast. But at other gigs using the cap did not sound appropriate so I used a "fatter" tone.
This could be adapted to literally any passive bass, of course it would really be most useful only on two- or three-pickup basses.
