I was reluctant to answer anything because I'm not sure either...
I _can_ tell you that there's no crossover, just the capacitator stuff you mention so it's really basic.
I don't recall the connections being of a type that comes loose easily, so I don't expect that to be the problem either.
Which leaves you and me with...not a clue
All I can say is that you could try to do some testing:
- does it malfunction at all volume levels? (could rule out connection problems)
- did you try and pry off the grill? (although hardly anything can be seen from there)
- and last but not least, you could (and would have to) remove the door panels so you can access all wiring and/or remove the tweeter (it cannot be done from the front, unfortunately).
There have been some posts on removing the door panels before. Do a search for 'remove door panel' in the archived posts for the Audio section and you will run into the procedure, even with some pictures (or links to them).
Good luck. I'm curious to know what might be the cause, so keep us all posted.
Edo
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(2003 Silver 1.4 Tdi Exclusive(SE), 95bhp/235Nm chiptuned by ABT, black interior, sport seats, heated mirrors, TomTom3 PDA Satnav, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)