Oh, you'ld like to know how ?
First get the GM "interrupter" (magnet -not at home so do not have the p/n handy but was $11/$14 last year.. You will notice it is shaped like a top hat with two ears sticking out. The approved factory "snap in" way is to insert into the cam sprocket from the back until the ears pop out in the machined recess. The hard part is that the sprocket must be off the car to do it which means the timing cover must be off.
(Note, if over 100k or the timing chain feels loose if turning the balancer or an 88 or earlier, this is a good time to replace the timing chain and any worn sprockets)
However if pressed for time there is a way to replace the interrupter through the sensor hole and you only need to remove one bolt to do it.
Take the GM interrupter and grind or sand away the brim of the top hat leaving the cylinder only. Try not to damage the two arms that stick out. Do not clamp so hard that the plastic breaks either.
Next remove timing belt and the cam sensor and rotate the crank until the magnet (or more likely a hole with some plastic remnants - brim is probably still there wedged between the cam and the sprocket) is visible (see .jpg at bottom).
Using an awl or small screwdriver clean all of the excess/broken plastic & gunk out of the hole (this is the most time consuming part. Use a mirror so you are sure to get all of it removed. This is critical. Will be able to see cam through hole when done. Then clean thoughly with carb cleaner or something similar that won't leave a residue.
Take a fresh tube pair of JB Weld (that is what was recommended to me & I have no reason to change) and mix up a batch. Apply liberally around the sides of the interrupter but leave enough of the top free to hold.
Now comes the tricky part - inserting the ground down and JB Weld coated interrupter through the sensor hole and into the now clear hole in the cam sprocket. I used a 1/4 drive socket (forget what size but just larger than the interrupter diameter, packed the socket so that most of the interrupter stuck out, and put on a 3" extension - you might want to experiment with this before mixing the JB Weld ). You need a way to hold the interrupter tightly enough so it won't fall out while you push it into the through hole but loosely enough that you can pull the socket back out without the interrupter coming with it.
Once started in the hole, remove the socket and push it in with your finger until you feel the arms bottom in the recess (will see what I mean when cleaning the hole). Make certain it is in far enough to clear the timing cover (not by much) when the cam rotates. When seated all the way, it will be.
Finally schmear some more JB weld around the part of the interrupter sticking out of the sprocket taking care not to get any on the top of the interrupter - probably not necessary but won't hurt.
Once done, let it dry with the cam sensor off so open to the air for the full period marked on the JB Weld tube. DO NOT try to start the engine "just to see" until then.
When completly dry, replace the cam sensor & timing belt and go. .
Like anything else automotive, care and preparation is the key. That said I did one a year ago and have had no problem since.
Can see a picture of an uncleaned hole with the brim still in place at http://www.6007.us/cammag.jpg
Also the interrupter mod: http://www.6007.us/cmmag.jpg