There are some really cheap cameras out there that'll do video respectably (keep in mind the length of your flights - if you can download them after every flight, you'll only need about 3 hours max, to be safe - that means you can probably go with flash mem, which is cheaper). From there, you can use an
automotive camera mount (a car club I'm in uses this one for road/track days), provided the aircraft seats have headrests. That'll mount the camera between the two seats. If you want radio chatter, you'll need a video camera with a Mic input. If you're doing dual training and the plane is only two place, you'll need a
double headphone adapter to run from one of the headsets. Then you'll need an
adapter cable for the mic input on the camera. The last link is a cable with a 1/4 jack on one end and a 1/8 jack on the other, so you don't need any jack adapters, and it's 6' long, so you shouldn't need any other extension cords.
There shouldn't be a need to run a mic cable from your headset to the camera, either. You mic input to the radio is re-broadcast on the radio output. It's how you communicate pilot to pilot, so the camera will pick it up.
