130amp highlander amp is a direct bolt in.
I ran my 1200w JBL without much dimming after upgrading the big 3.(to leftover 4ga wire) Then again HID's don't dim b/c the ballasts fairly well even out spikes and they use what? 3-4 amps less power?
Anways... Upgrade the big three, that is very cheap, and will slove, maybe 40% of the dimming. Buying a stronger alt will take care of the rest.
Instlling subs is not difficult. Do you have a head-unit with pre-amp outs? if not, you can just splice into the left and right channels for the 6*9's behind the back seat and run them into high-level inputs.
You route the power cable throug the stock harnesses into the cabin.
Follow mine. You jack up the front, drivers side of the car, and take the wheel, and wheel well cover (the plastic thing that covers the inside of the wheel well) out.
You punch the amp wire in with the factory location into the wheel well. (you will have to slit the rubber seal to fit the cable in)
You then follow the stock wiring into where it enters the cabin (it's behind the fuse box on the driver's side. Waaaay back there) Slit that rubber seal and pass the cable through.
Then you just pop out the interior door trim bits and route the cable at the edge of the carpet back into the trunk.
Sounds complicated, it's really easy. You just have to ge started and look at it, and it will be 100% understood.