Is there a built in amp that will only work on a stock HU on a '94 maxima?

I hooked up an after market deck and now only the front speakers work. I
have tried a new harness, a new deck. Still the same thing. I am positive
that the deck is hooked up properly. I have installed decks in tons of
Honda/Acura and have never seen this happen. Someone was telling me that
Maximas have a built in amp. This is not a Bose HU just a tape deck if that
helps. If I put the stock HU back in rear speakers work.

Does anyone have any suggestions?