Truck ground

It's entirely possible not to get a good ground no matter what part of the chassis you connect the wire to, but is seldom the case. 

The ground wire has to be at least as large as the power wire and needs to connect to the source ground.  The source ground in a vehicle is ultimately the battery negative, and it connects directly to the truck's sheet metal chassis.  If you would read the "...upgrade the big three..." sticky subject above, you will see that this battery-to-chassis ground wire may need to be upgraded.

The sticky subjects dealing specifically with "...ground..." should also merit your attention.  Basically, you want to have good, solid, well-connected sheet metal panels between your amplifier's ground location and the battery-to-chassis location.  Look at the sheet metal as if it were a wire itself...if there are breaks in it, it won't transfer current very well.  You can ground directly to the battery-to-chassis bolt just as well as grounding to any other chassis spot, in which case you are substituting an actual wire for the chassis.

Best grounding is done when you drill a hole instead of using a OEM bolt...scraping and grinding all traces of paint and/or corrosion from the area...using a ring terminal on the ground wire that is  well-crimped and well-soldered onto the ground wire...and using a star washer at the grounding point  (digs into the metal).

So, pretty much anywhere on the sheet metal will work?, I bought a kit with 2 feet of the ground wire, or else I would just run it to the - terminal on the battery

Seat belt studs are sometimes a very poor grounding location, as they generally use hardened steel bolts and nuts welded into the sub-frame.  Some seat belt studs have 100 or more ohms of ground return path resistance and will provide a nice source for ground loop hum.  It is always best to measure the ground return resistance before using any pre-drilled ground location.  I believe these topics and more are covered in the Ground sticky posts at the top of this forum.  :)