Will the JBL amp accept speaker line in from an aftermarket Headunit?

I bought the car with the original JPL amp and speakers still hooked up and working; however, they had replaced the factory radio with an after-market double din Pioneer. Everything works, including steering wheel controls. I just have a question about HOW it's currently wired. Nothing is plugged into any of the RCA's on the back of the Pioneer and the speaker line-outs from the pioneer are spliced into a harness that mates with a factory harness. So I assume that the JBL amp is getting it's signal from the Pioneer speaker outputs. Does that sound right? Just trying to plan an upgrade path.

Thanks!