Isn't the RCA an output for an equaliser or pre-amp or amp? IE - it will not power speakers.
Usually your HU with RCA out feeds the Eq which then feeds an amp that feeds the speakers.
The amp could be the HU if it has RCA out and RCA in specifically for EQ and signal processing etc (ie RCA out can feed the RCA in). Don't confuse that with an auxiliary RCA in which is before the RCA out.
I'm not sure splicing speakers into RCAs would work an any world - except perhaps my old world. AFAIK, RCAs are now low-signal feeds; speakers are different or bulkier connections.
Self powered speakers are a different issue - but they are usually RCA or Philips input. In those cases, buying adaptors is probably worthwhile.