If you cut open the wall (remove some drywall) in the garage and do your wiring and then replace the drywall ( which should be 5/8 type X for fire code ) tape and mud the seams you should be back to code compliant. You do not want to make any permanent openings in the garage wall that fire can pass through into the living space.
When you cut out the drywall in the garage you want the vertical cut on the center of a stud so that you have structure to reattach to when you put the old/new piece back in. You can do it with a razor knife but i find an oscillating saw to may much quicker work of it. Find your studs ( since you garage is not finished you should be able to see the drywall nail/screw marks on the wall ) Use a level as a straight edge to mark the lines for you to cut out, When you cut your horizontal lines do not plunge the saw deep into the wall cavity as there may be wiring, pluming etc in the space, run your say at an angle so that just enough of the blade it in far enough to cut the backing paper of the drywall. On your vertical cuts your blade will bottom out on the stud but you may hit the drywall nail/screw heads, move over these and leave the drywall around them to be cleaned up with razor knife.