Thread title meant to say RCA splitter!!

I have one RCA cable running from headunit too 4 channel amp, its possible to use the amp in 3 channels, i want to run 2 channels to a pair of componants and 1 channel to a sub. This is achievable by having two RCA inputs on the amp, Channel A and Channel B,
Channel A will provide Audio for componants, Channel B will supply Audio for the Sub. Can i just stick a splitter on my current RCA and split it for the channel A&B or shall i run a complete new RCA from the headunit, there is a RCA output dedicated to a sub so i get the feeling a complete RCA might be the answer.
Advice please? I dont fancy ripping out the interior to run new RCA's and i already purchased the splitters