sorry I was busy, couldnt reply sooner.

I guess I will go through the false trunk floor with additional lining below it. That should keep it secured. Thanks, I never thought of going that way.

My ground is on a screw behind the back seat towards the passenger side. There was another thin wire grounded on it. I guess it is a cover for something though, the screw is on a circular plate. Even removing the screw that I have chosen as ground doesnt move the plate.

I chose that ground because I didnt want to drill a hole in the trunk and I have least knowledge in drilling in the car. With the gas tank and brake lines running there, it was a big no no. Also a previous ground cable to that screw made sure that it was a proper ground.

the back seat is a poor ground point. the seat is bolted to the floor, but it's painted metal. there is a lot of resistance in that connection. as current increases, that resistance is very important. the small wire has low current, so it's not as much of an issue (though still not ideal)

maybe look to the side of the trunk, or support bars that are welded as opposed to bolted.

Oh, no its not the back seat screw that I grounded the amp on. I grounded it behind the back seat on the trunk floor, on a pre-existing screw that had another thin wire grounded. And I scrapped off the paint with flat head screw driver before connecting the amp ground. I checked the ground with a DMM with the amp connected, disconnected and turned on with the music on/off. All the time DMM showed above 12.4V.

This weekend I am going to get some brackets and bolt the sub box on the false trunk floor. The amp is mounted to the back of the sub box for now.

Thanks for all your help.
I posted another thread for installing driving/fog lights, I am sure you will help me there as well.

ah, good to know it's on the floor - good call.

As I said earlier, I am no good with drilling, so I chose the existing bolt.

However, strange thing happened today morning while I was driving.

I was listening to music from a USB drive via my sony xplod deck, the amp was running fine and the volume was not so high as well. When I turn on the headlights, suddenly the deck went off and kept on restarting. I thought may be I blew the fuse since I was using headlights, blinkers, amp, but it shouldnt be the case.
After I parked, I started the car again, and whole day the deck is working fine, no restarts.
I am not sure what may have caused this. May be just a random thing, but just wanted to update.

Sounds like a short?

I dont think it should be a short, because I just installed it a couple of weeks ago and I made sure everything is connected and insulated properly. However, I will check the wiring and the fuses tomorrow, since its a weekend and I have nothing interesting to do.

maybe low voltage. get a volt meter at your dash. even the kind that plug into the cigarette lighter.

I have a Digital multimeter. But I do not know what you want me to measure in the dash?
Just a note, the head unit restart again once today.

a DMM is nice, but you want to monitor the battery voltage while you're driving and the HU is acting up. could be that it's a Sony and they fail often. could be low voltage as a result of poor grounding. could be a short in the wiring somewhere.