V40 Factory Sub and Amp with aftermarket HU

[sandman]

Hey folks, I've been looking for ages trying to find this information, and coming close, but not really exactly what I need.
I'm wiring in my aftermarket HU (replacing a different HU from previous owner), I'd like to run from the RCA sub out line level outputs into the factory amp/sub system. I'm thinking my options are going to be:
1. run RCA all the way through car to wherever the amp is, or,
2. run cables from the HU behind dash into the 8 pin din plug? will this work somehow to the amp, or this only from the factory HU??