I use a volt meter and tap the +12 volts then start poking around where the amp is until I get a good ground

 

Just a little word of caution on finding ground spots
I did the same thing with the DMM in one install. Showed 12v fine without a load. When I tried to start an amp it dropped to about 5v. Spent 2 hours trying to figure it out.
It was a unibody car and that piece of metal I wanted to use for ground may have not had a good connection to the rest of the body.

 

On mine I just used a self tapping screw with a washer and put it in the floor. Worked perfect for 5 years now.