If you can reach in there, I've always taken a sharp nail punch and beat it a few times, then you can feel for the dimple on the other side and make sure nothings in the way. Best to use a grommet and some silly cone to get it nice and water/air tight.
Good luck and don't be afraid to drill :-P
Chris
PS if you do find a rubber grommet here is the old trick I learned, get a radio antenna (old sold non power one) Grind one end really sharp and tape the other end like a handle. Attach the wire to to it with good electrical tape, wrap it serveral times TIGHT and then lube it with some KY Jelly (dont laugh I still have some in my tool box hehehe) and viola you poke it right through.
But remember always wear protection... I mean eye protection...
Chris
Can't ever keep a straight face when telling people to put ky jelly on their rod and shove it in the hole.....