The PAC OEM-1 has gain adjustment knobs which must be cranked all the way up. At least that's the way it was on the one my friend and I installed in his Durango. Did you turn up the gain adjustment knobs?

Even with the knobs cranked, it still isn't as loud as his factory radio was. I think this is the fault of the PAC unit, it should have a wider range of gain adjustment for line-level devices if they're going to offer it as a Line-Level-To-Infinity-Amp solution like that.