What kind of amp is it?

The JL Audio 500/5 I recently sold to a friend has all of the input connections you described, but only requires one input channel to provide a signal for the entire amp. The more input channels you can give it, the more control you have over the sound. But if you only have one, it'll pass (share) the signal from front to rear to sub.

I'd suggest reading the amp's install manual.... or at least tell us the brand and model so we can help.

JD