2003 Hiace Rear Speaker Wiring

[ARHiaces]

Hi all, 

Adding in some rear Speakers to my 2003 hiace commuter (12 seater)

I’ve recently purchased a JVC head unit with the appropriate harnesses (JVC to ISO & ISO to Toyota). All hooked up and working properly, but I don’t know where to wire in the rear Speakers. The van only has 2 small front Speakers, I’m assuming there is 2 more channels ( 4 terminals) somewhere in the harnesses for rear speakers too? Or do I have to splice them into the existing speaker cables?
 

As you can probably tell, I have next to no experience with electrical/audio stuff… any knowledge or advice on the matter would be much appreciated! 

Andy