1999 Galant Aftermarket Stereo Installation
 

Stock system is removed
Previous owner connected head unit to separate speaker on the floor (read/front speakers no sound)

I purchased a Duel XD1222 (cheapo) unit:

I took it to CarToys and they quoted me $150.00 (because it has an Infinity sound system). They noted this is the reason the current unit is connected to a separate speaker as the other owner "did not know what he was doing".

In my initial research, it appears that I need one of two things:
Metra 70-7003 Mitsubishi Amp Integration Harness
or
PAC C2R-MIT Infinity Radio/Amp (MITAH)

Which of those two will work for my unit? Thanks in advance, your responses are duly appreciated. Also, if there is anything else I need to know before I get going on this, please let me know. Thanks!