For some reason my mic input on my 5D Mark II isn't working... maybe I am just missing something. I tested a mic with my mini disc recorder and it worked fine, but when I plug it into my 5D I don't get any audio from the mic.

Anyone have any thoughts on this?

I'm shooting with movie display.

Specific details on the mic, please?

Does the mic require phantom power or a battery?

Are you just listening to the clips on your camera or have you copied them to your computer and listened? I ask because I had the same problem, I connected a Rode NTG-2 using an XLR to mini adapter and when I would record a clip and play it back there was no audio despite the fact that I had the volume on the camera maxed out... anyway, I copied the clips that I thought had no audio to my computer and the audio played just fine on there. I still don't know why the audio recorded with an external mic doesn't play back on my camera though... recorded audio with the internal mic plays back just fine.

It actually worked with one microphone that uses a battery, but I didn't have a battery installed.

I also tried using an SM58 and that didn't work.

In one clip i wiggled the cable and it cut in and out... so i might have a faulty cable or input jack.

... so i might have a faulty cable or input jack.You could test it by plugging the cable into a PC soundcard mic input, and attempting to record into the PC.