Thanks I wasn't aware about connecting the negative to the car instead of the battery so I will do that now. I believe I understand what you are saying about the Ohms Impedance I've checked my amp and it says:
1.5Ω = 1000w
2Ω = 1000w
3Ω = 1000w
4Ω = 1000w

So as its the same power that is supplied on each I will run it at 4Ω so I don't fry the amp. Someone also told me the higher the Ω the better quality sound as it's not distorted?

I have watched this video by BSW - BMW X5 Subwoofer System (x108) by BSW - Install Guide 3 of 7 - YouTube and they run the input from the existing OEM subwoofer connections (Option 1 in my original post) even though this is Post amp would that be the best option then? Does it not matter about this clean feed I heard about?

Would I need something like this to convert to RCA cables? - Stinger Electronics SGN11 Car Speaker to RCA Phono adaptor | eBay

Thanks!
James