Removed my old sub from the car and putting in a different one. I have 4 wires which I’m assuming need to be joined to make two pairs which I will then put into two RCA plugs to go into the input shown on bottom pic.

Which colours go with which?

Surely you only need to input into either the pre-amp or the amp part of the box, not both?

Most subs only have a mono input too don’t they?

This is what’s on the other side. I’m assuming this bit connects to the sub with speaker wire.

was the box they came off labelled?

you need to find out which is which at the other end. Easiest way would be to note how they were put into the old amp. You did take a photo didn’t you?

Edit – I’ve just looked at your old amp input.

Assuming those are speaker outputs, not RCA at the other end from the head unit, they go into the ‘high level’ inputs, NOT into the RCA, you’ll break it if you do that.

Yup, those look like speaker level inputs, your amp will need to have those.

So.i need to get some sort of an adaptor to stick them in the high level input socket?

Yup amp would normally come with it if new.