Will a 99-00 headunit fit and plug in to my 96 civic?

 

Nope. Plugs are different. Physical shape of the front faceplate is different (the 99/00 is a true rectangle, whereas the 96-98's had that weird angle on the left side).

 

that sux, thanks for the info

 

You can make it work, however, if you get a harness for your '96, and wire it to a reverse harness for the 99/00. You'll need part numbers 70-1720 and 71-1721 (Metra). Match up the colors and plug them in.

However, the deck won't fit right in your dash, due to the shape of the hole on the left side.