I recently installed a 2 x 10" Subs and a 600 WATT amp in my 99STS.
I kept the original stereo and everything else is original.

Sounds great but sometimes the volume will jump up (and down) really quick without me touching the control. the guy who installed it told me it might be due to the sensors or "ambience" in the car and that the Bose system is compensating the volume in the car.
is this true?

Does anyone know anything about the Bose system and what is the best setting (i.e. Ambience, Front Seat) . I currently have it on Ambience with the level set exactly at the middle. its fine like that most of the time.

I think there is a way to turn-off the auto-volume.

yes.
its the auto-volume! thats gotta be it.
thanks dude. i'll figure it out.

Well turning off the auto - volume did the trick.
thanks.

Shit man.. i never knew that the battery was in the back seat.
now i got to rewire my system.. oh well.

I figured that would do the trick. Good Luck on the system!

Hey how did you hook up the subs I thought there was no sub output?

I hooked up 2 12's, and tapped a line out box into a rear speaker, which provides a location for RCA cables to feed into the amp. Interior looks stock. I think the boom from the trunk does not sound stock, tho!!!

Should the battery just be out in the open under the seat like that? I heard it was supposed to be in a box or something, you know in case it decided to explode. Also, supposedly there are gases that are supposed to be vented outside by this metal box.... this is my 99 STS as it looks right now. Anything wrong?

thats the way mine looks also.

do you have any pics of the sub and amp install?

how does it sound with the stock radio?