How can i thread a cable for my amp through to the cockpit from the engine bay of my BMW e36??

If you look at the bulkhead, you'll see that there are numerous holes already there with wiring passing through them. It'll probably be a fiddle, but you should be able to thread a cable through one of them.

dont know about you but my battery is on the left. the easiest way is to get on your back down there in the foot-well. should see a wire loom already going through there. some cars have spare grommet holes..it is not likely that you have to drill, but then again it is not unheard of. if you really have to drill it is best to use some Vaseline/grease/silicone before pushing a ring grommet back in. also use a vacuum to pick up all the fillings. O.. one last thing make sure you run the speaker cable and the power cable down diff sides of the car.

You need to drill a hole, insert the proper grommet to prevent causing a short and seal it with GE silicone sealent. Proper way! Or ask the good fairy to carry it in.

If you look behind the mat behind the glovebox you will see a couple of rubber grommets, all you need to do is poke a hole in one of them