Sound System Upgrades

Hey guys, I'm new to the forum and this is my first post so if it's not up to par with everyone else's I apologize in advance. I recently began an aftermarket audio system on a relatively low budget. I have bought all the components, and everything is in working order and installed properly, however my 2 subs will not make a sound. The 2-channel amp is on, all of the different settings to no avail. The amp is rated at 400 watts, and my worry is it's not strong enough to power both the stock speakers and my aftermarket ones. Any tips/ advice would be wonderful Thanks!