I also have a reg cab, and I have to get the new series JL 10's. Ultra thins then had a custom box made for them. I have 2 behind my seat

 

yea i have an ext cab and i just lifted the seats and put the sub on oneside and the amp on the other...im 6'2 and thought it would be fun to ride in the back seat of my truck while someone else drove...nope...i was suckin my knees all the way home im almost thinkin bout just takin them out if thats possible

 

So the original question was finding a good ground, correct? If I'm not mistaken, as long as you are hitting metal, it's grounded (i.e. neg or -). Look at your battery, it's connected to the frame on the Neg or - side of the battery. The entire metal frame and body of your vehicle is a ground. Frame or body (sheet metal) is a ground. Stick the amp where it will fit, screw a ground into metal, and you should be good to go. If there's any question, test it with a meter.
I'm not trying to be an ***, just trying to answer the original question. I gathered he wasn't asking WHERE to put it, just where to ground it.

 

Though if you have a really powerful amp, you may want to route a ground directly back to the battery. Switched of course. If not , you take the chance of draining your battery if you forget to turn it off. Food for thought.