1997 Jeep Grand Cherokee Amp Install

General rule of thumb is to keep it at least 18" away from any RCA cables, although for obvious reasons the further away the better.  Since the turn-on wire handles a tiny amount of current (500 mA at most) it is not likely to cause interference if run next to the RCA cables.  A lot of installers prefer to run the turn-on and the RCA together because they both originate from the same point (the back of the head unit), and it makes it easier to tape the bundle together.  Usually I run the power cable down the same side of the vehicle that the battery is on (usually driver's side); and then the RCAs go down the other side of the vehicle.

Since you're putting the amp under the back seat, what you should do is run the wires along the edge of the carpeting inside the car, just enough to hide them and prevent anything from catching on them.  Then, once you've run them far enough so that they line up with the rear seat and where the amp will be mounted, you can then change directions and start running the wires inwards towards the middle of the rear seat to where you'll put the amp.

Running them like this will keep the RCAs as far away from the power as possible for most of the run of the cable.

Try to find an existing grommet in the firewall that you can widen just a tiny bit so that the power cable can run through there.  If you can't find an existing one you'll have to drill your own hole (check both sides of the firewall multiple times before drilling!), and use your own grommet.