Here is a very good article that outlines how GM's TCC (torque converter lockup) logic and design has changed over the years.
http://www.motor.com/magazine/pdfs/062006_07.pdf
The main reason for posting this is to learn how OEMs do it, so that we can modify our own transmissions to do what we want. Simply adding a TCC lockup switch in may not work. Its also very likely to cause a check engine light on more modern transmissions.
Has anyone installed a TCC lockup switch on a newer vehicle? Even my 1997 Sunfire has a PWM solenoid instead of an on/off solenoid. If I put a switch on, I expect a check engine light.