CAR Amp high output to speakers alsop need RCA for a sub

High, I am using an amp and it has front and rear .  I am using the rear  high output into low low level converter to put RCA to a sub.  I now want to use the rear to power speakers, but I will need to RCS to the amp, or ligh level to amp. 

can I split off the high level to speakers and still use the low level converter without changing the ohm?  

Will I berak anything?  is there a way I should do it?  do I have to buy something?