we cannot find the factory amp,not under the front passanger seat??
Then you don't have a factory amplifier. Under the front passenger seat is the install location. Someone may have put a newer stereo unit in with more power. I put one from a 2004 in mine (with a little mounting modification) . The newer unit had much more power then the 99 did. Someone may have added an amp as well. I would dig around in the trunk. !~Good Luck~
The JX did not have the amp as standard. JXi models had them with the standard infinity systems. You may be better off without it as the underseat location was prone to moisture damage.
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