I put the battery on the wrong polarity briefly and blew the computer and the stereo went out too after replacing the computer all problems were fixed except the aftermarket stereo is still dead where is the fuse for this or could it just be fried?

Most aftermarket stereo units have an in-line fuse fitted either on the live wire at the back of the unit, or fitter directly into the unit. Remove the unit and you will see it...no problem.
However the bad news is...in all cases that I have encountered where a battery was connected up the wrong way...the unit was fried...deep fried. Way too badly burned to repair. But I wish you all the best of luck with it.
John

For an after market stereo the fuse should be an inline fuse in the power wires that the installer ran from the radio to the fuse box. May be a fuse in each wire. Red wire and yellow wire. Also check and see if their are any fuses in any of the wires ran to the positive side of the battery. I am unsure if reversing the battery wires would blow a fuse or just fry a unit.

It should be behind the stereo