Anyone tried running the rear speaker cable into the high input jacks on an amp to run a sub then just splicing the same input wire back to the rear speaker. Does this screw up the sound? It would be on a 06 shaker 500 system.

 

If you run a cable from an amp to the high input of another amp, you are, in effect, squaring the amount of distortion and, most likely, screwing up the sound. It may be an easy connection, but the sound will most definitely suffer. IMO, replace the entire Shaker 500: cheap head unit, cheap amps, and cheap speakers all around.

 

I think you would be better off getting a speaker to line level adaptor (radio shack or car install shop). They have adjustments on them that allow you pad down or increase the input side to give you the proper line level signal to hit the line level input of the amp. I did this in my F150 and it works great. Just splice off of the speaker cable as you mentioned, into the converter, and rca jumper cable to the amp. And no it doesn't hurt to splice/Y that speaker cable to the converter while still powering the speaker.