89 mustang - factory amp?

I am familiar with this car and I can answer most any question you have. As far as the harness you have you will not be able to use it unless you bypass the amp and I would have to explain to you how to do that. It is a 70-1770 I presume? Now as far as keeping the amp, Metra makes a harness for this vehicle to keep the amp if you so desire but I would'nt keep it. That part  is 70-1768.

Now as far as the speaker impedance go's, don't worry about it, it will fine powered by the Sony deck. Let me know what you want to do.

I cant remember for sure, but on some mustangs is if you look behind the radio or behind the glove box toward the radio there is a second harness that goes from the harness from the deck & factory amp to a plug that goes out to the speakers, which uses the standard Ford harness, if this is the car that uses that, all you would have to do is unplugg where the radio/amp harness meet the speaker out put and plug the harness  you wired to your new deck into that. Ive done a mustang like that before, took a while to find the harness hidden behind the dash but afterwards it was just like doing a basic deck

Yes, the plug kit I bought is the 70-1770. I pulled the stereo out last night and tried tracing some of the wiring down, I'll have to look again to find the second harness. The only ones I saw were on top of the amp and were what looked like 4-wire, in a square pattern as opposed to the single-layer flat connector like what goes into the back of the radio. My personal preference I would like to bypass the amp if at all possible, I used to have a 93 Ranger that had the JBL premium sound and I ran into this problem with it. The factory amp exaggerated the volume so much that whenever I would unhook the battery my radio would return the volume to somewhere around 10, which eventually blew my door speakers. I bypassed the factory amp on it and it solved everything.

From what it looks like the 70-1770 will snap right up to the connections that I unplugged from the back of the radio, so what I'm gathering is there's another plug somewhere back in the dash that is similar to those that I want to connect my Metra kit to? Also, I'm trying to figure what you mean when you mentioned the front speakers negatives are reversed. I'm assuming you mean reversed between the left and right channel, or do you mean polarity?

Just out of curiosity, does anyone know if the front channels on this car are set up like component speakers with a crossover or are they just wired together? Just curious as this seems like such a mystery to me, plus I noticed that absolutely no bass comes out of the dash speakers, it sounds like there's some type of crossover or filter on them.

Thanks for all the info, my goal at this point is to have it installed next week pending whenever my new Pioneer for my S-10 comes in. The Sony that I have in it now is going into the Mustang once it shows up, but I started pulling it apart last night after reading through this so that I can plan out how to do it. I'm sure I'll be on here throughout the week at least to update my progress on it, but probably with more questions.

All Done!

Got it all hooked up and it sounds pretty good, couldn't have done it without this site! I found the plugs that were mentioned back behind the glovebox so I unplugged them and then took the factory amp and the amp harness out, what a ton of wiring for a stock radio! The kit that I got for it turned out OK even though I had to cross a couple wires to compensate for Ford's reversed front speaker negatives. It fit the plugs that I found behind the glovebox (which looked the same as the plugs going into the back of the stock radio), so I plugged my Metra kit into those plugs. I got all six speakers working just like it did before and it actually doesn't sound bad at all for stock speakers. Tech460 I figured out what you meant by the reversed speaker negatives when I took my diagram I printed off of this site and traced each wire down, bigtime thanks for the heads up, saved me a lot of trouble and frustration as I wouldn't have even though to check that. I just can't figure why Ford would use those plugs and reverse two wires in it, seems kinda goofy and real easy to screw up when going aftermarket. Install looks real clean too. Thanks everyone again for all the help.