aftermarket radio

[MikkiJayne]

Ok so it looks like the active rear speakers are driven from a small yellow connector on the back of the head unit - this is usually part of a block of three small ones which clip together making up a single long one. If you don't have this connector then I think you must have the fully passive system in which the rear speakers are driven from the head unit as you expected. If that is the case I'm not sure why you wouldn't have any output from them.