So first things...assuming this a UK 10th Gen Civic, with the 11 speaker system? My install is on a UK 2019 Sport Plus with "Premium" audio (the 11 speaker system).
Firstly there are no speaker connections at the headunit itself on the 11 speaker system AFAIK. There is a digital connection, a single cable carrying an S/PDIF signal for all the sound data, between the headunit and the amplifier located on the side panel in the passenger footwell.
The actual connections to the speakers are from the amplifier on a multi-pin connector, so you will need to pick up the signals from here. As you have found there is no RCA output so you will need a line out convertor if your subwoofer does not accept speaker level inputs.
Apparently the signals to the rear speakers are not full range so you should take the signal from the front speakers.
These are the colours/positions/and functions on the multi-pin connector of the amplifier for my car for the front speakers.
Front Left Tweeter Positive - Position A14 - Yellow at Amp - Yellow at Speaker
Front Left Tweeter Negative - Position A15 - Green at Amp - Green at Speaker
Front Right Tweeter Positive - Position A16 - Light Green at Amp - Blue at Speaker
Front Right Tweeter Negative - Position A17 - Pink at Amp - Pink at Speaker
Front Left Woofer Positive - Position A5 - Light Green - Pink at Speaker
Front Left Woofer Negative - Position A6 - Light Blue - Black at Speaker
Front Right Woofer Positive - Position A12 - Purple at Amp - Red at Speaker
Front Right Woofer Negative - Position A13 - Grey at Amp - Black at Speaker
I take no credit for the following images, they were found on this thread which was very useful Photos of Touring Amp
. Note that this shows a subwoofer output, those positions will be empty on UK car (as we can't handle the bass apparently
.) In addition I think the A12/13 connections polarities are wrong on these diagrams - i.e. purple is positive, grey is negative, hence the difference in my notes above,
Hope this helps.