Well Czar depending on what it is you are doing this for.  If you happen to have some lightweight chain or my personal favorite lead sinkers (fishing weights) they both work well.  The chain one takes a little more finesse.  If you got any questions you know where to find me.

Ok this will make it nice and simple.  Pop the ceiling tile closet to the wall (I know duh).  Find where the insulation reaches the ceiling or find a spot that is between two pieces of insulation.  Take your cat5 cable and tie the lead sinker to it.  If you have got it behind you'll hear it fall down.  Once it is down pull the other end of the cable so you can hear where the line dropped.  If everything went right you should be right between those pieces of insulation again.  Now depending on how big your box is (insert pun here) you should be able to at least get a hand in there, might not be yours sausage fingers.  Now you will either need to find where the insulation meets the floor or the spot between the 2 pieces of insulation.  This can be a real easy job or one bitch of one.   Your best bet is to try to get behind that insulation but try it without first, if the sinker is heavy enough it should drop through that wonderful fluffy fiberglass.

If you aren't rushing and trying to do it right you should be fine.  But as with all small jobs they can turn into a nightmare.  Don't hesitate to go heavy with the sinker weight.

Use a chainsaw. The end.

Glowrods are really what you want for this but yeah.