Headunit in a stang?

You have to buy an adapter plug that plugs into both harnesses and then comes into one for your head unit of your choice.

I went with this then get a dummy pocket and place the radio where you want it on top or bottom.

I had this question when I got my head unit. The stock unit is huge, and I was wondering how the heck they were going to make it look good when the aftermarket Alpine I was getting was about 1/4 the size. What they did for me (had my system professionally installed) was this: They mounted the aftermarket deck on the top side and underneath it, they put a dash kit in which is basically a piece of black plastic that you can throw your cd cases in or whatever. It looks really good now, there's no gap at all, and I haven't had a single problem out of it yet.

It's called a dash kit.
Just go to Best Buy and tell them you need a dash kit for a pre-2001 Mustang. It'll basically delete the 2nd opening by turning it into a storage space.

2001+ Mustangs use a dash kit that adds and small opening above the new headunit. The opening is pretty much useless. It's too shallow for cd cases I think I used to put my gate-card in it and that was it.

The real fun ones are for Escorts and Tauruses. Those dashkits include switches for the a/c, since the Ford stereo and A/C controls are all one unit.

Just make sure you get the correct one for your year. But at the same time I'm just wondering if you should look into having a shop do the work.