How does an Air-Thru stock work?

Why won't my stock tweeters work after hooking up an aftermarket 4 channel amp?

• I have a 2006 VW Jetta, and I hooked up an Alpine MRP-F240 4 channel amplifier between the stock radio and stock speakers. Now once everything is connected, all 4 doors work fine and sound great, except the tweeters in the two front doors no longer work. The front has stock 3-way components, the mid and low work fine.theres only one set of wires for the whole door behind the dash and they are hooked up correctly. I tried changing the speaker level inputs to a line out converter, and I'm using a LOC on my other amp (sub, works fine), with remote on for the Alpine amp. Does anyone have any thoughts? Is this a can bus or an ohm (impedance) issue? Is there any way to get power to those front tweeters? The rear has a tweeter and a low that work fine. The door panels were NOT removed.

• Answer:

  The 3-way speakers have crossovers in them which may be blocking the highs that would otherwise play the front tweeters.