Line level converter

I'm adding a sub to a factory system and need to get signal to my amp, which has line level inputs.  My question is, will it damage my head unit to run signal to the amp and to the rear door speakers?

No, it will not damage the deck.  The speaker wires carry a voltage which is picked up by the LOC, and that voltage remains constant whether the LOC is attached or not.  The LOC does not pick up the current that is travelling through the wire which is what is feeding the attached speakers.  So your rear speakers will get the same output from the deck in either case, and the deck doesn't suffer from an added load.  The voltage is free for the taking.

If i'm reading what you're saying correctly... i can use a 4 channel LOC on a factory deck to convert the speaker level output to line level output (basically creating 2 sets of preouts, Front and Rear) without having these speaker level outputs directly attached to the speakers .... and i can do it this way safely without worrying about damaging the amplifier in the factory head unit?

Yes.  A resistive load on an amplifier will cause it to output power.  When speakers are connected it will produce amperage;  when speakers are disconnected, it will stop producing.  Ohm's Law.  Either way, it still provides a voltage which is usable by a LOC.

The LOC is not a resistive load, but a signal-sensing device.  Its attachment to the wires does not add to the load presented by the OEM speakers.  So, you can leave the OEM speakers connected and still use the LOC if that is what you want to do.  Usually this is not done because the amplifier you are feeding signal with the LOC is replacing the amplifier in the head unit.

A deck will provide a signal source (the voltage) whether there is a load of speakers attached or not.

Thanks for that. much clearer now!

Any suggestions on a good 4 channel LOC?

http://www.davidnavone.com/adaptor_products.htm

Use the NE-774V.