IIRC I remember seeing a harness for the car floating around, for non-NAV cars only.

But I have no clue if it's still in production or any place to buy it that isn't unbelievably shady.

Best of luck with your project sir.

 

My search failed. Here's the amp I tried to use part number 86280-50160 says out of a 1998 LS



As I said could not get good sound out of it. Also keep in mind the amp has no sub input there is an internal filter that splits the frequencies and sends the low signal to the sub. This isn't ideal because it means you will have less control over bass level. I will warn you a project like this is very involved especially if you have to cut/splice your own wiring. Not a matter of plugging a few things in and done, honestly it is easier and better to replace the amp as well.

 

The amplifier has digital circuits controlled by bus signals and it's not easy to override the control without analyzing the algorithm.