What didn't you understand?
If the amp has speaker level inputs, you can use them, but you will tap into the front or rear speaker outputs to feed the subwoofer - so when you fade to front or rear you will change the subwoofer volume.
Same thing will happen with a LOC unless you use a 4-channel LOC and then Y-adapters to combine the front and rear outputs to the subwoofer amp.
Best way to do this is to add a 4-channel amp with RCA pre-outs to feed the subwoofer amp, but it's easy and almost cheaper to replace the factory head unit.