starting to regret screwing with the Stereo -- AMP Wiring Question

[chief_wiggum]

Alright, so i had a shop install new front components, a 4 channel amp and wire up my passive 8" sub. after trip #2, the front components sound the way they should, but there is barely anything coming out of the sub (despite what should be 150 RMS being sent to it).

It's obvious that they don't have something hooked up right, but the reason I had someone else install it is because i'm fairly clueless. However, the amp currently has one pair of RCA jacks going in the input side, these are allegedly high level inputs coming off the back of the stock head unit. the switch on the amp is set to 2 channel. the speakers front speakers appear wired correctly on two of the channels and the sub appears to be wired correctly by bridging the other two.

Now it seems to me there should be another set of RCA cables on the input side and the amp should be in 4 channel mode, right? If so, isn't that pretty obvious?

 

[chief_wiggum]

Finally some positive news; to make a long story short, whatever signal the installer used to send to the amp is crap. When I hook my IPod directly to the input side of my amp via RCAs, the sound is great and what I expected it to sound like. So, now to get a good signal back to amp. I'm leaning towards the Pac aoem-maz2 since the installer still left the factory amp hooked to the rear doors and factory sub. Thoughts?