'99 GTP stalls once a week on me n warning! TCS light comes on, TAC dies then soon the engine dies! Not good doing +70mph in the passing lane!!

It is located behind the harmonic balancer.

~Larry~

http://autorepair.about.com/library/graphics/pontiac_cam_sensor_098c.gif

How difficult is it to replace!

Here is a thread that may help you replace the sensor and you should be able to pick one up from a NAPA dealer or most parts stores and dont forget that you will also need a case learn/CKP Variation Learn done after you install the crank position sensor.

~Larry~

http://www.automotiveforums.com/vbulletin/showthread.php?t=293188

CKP Variation Learn how to.

http://www.seattleautomotive.com/ckpdoc.htm