99 tahoe 2 amps

Well, never run the ground all the way back to the battery.  Most ground connections average about 3 feet.  Use the same gauge wire that you ran for power and make the connection to a solid point on the chasis - be sure to scrape / sand off any paint! 

They want you to run the wires for them, or is it already all done?   If you are going to run it, chances are you are going to use a fused splitter (called a distrobution block) to divide the 0 gauge up to two 4gauge connections, for example.  You can do the same for the ground, but backwards: two 4gauge to one 0gauge.

Let us know some more details!

Good advice.  I won't say never run a ground to the battery, but it should only be done as a last resort.  With that size wire, we can only imagine the kind of power the system will require.  Talk the customer into the highest output alternator available, and do the big 3.  Use the search function here (top right hand corner of the screen) and you'll find plenty more info on these topics.