94 ford f-150 lightening

My friend has recently purchased and the first thing the both of us wanted to do is take out the old head unit that the previous owner had put it, (not factory, aftermarket), the problem we have run into is that we are getting no power to the new head unit, there are two sets of wires as far as we can see, one which had a metra kit installed connected to a square eight pin connector with speaker leads, and the other connector was connected to power leads, so we connected the power leads, we left the speaker leads alone because he is running all his speakers and subs off of two amps, and no power.  then we disconnected the power connector going to the factory harness and plugged it into my 91mustang and the head unit powered right up and when  we connected my rca's we had full sound.  We checked his fuse box but no fuses were blown, the radio is an alpin cda7894, if anyone has anythoughts please let me know