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I can speak from experience with factory installed ones they are junk and are rather expensive to replace when they go out. Cool gimmick but still too new for me to try.
There are some leads in the archives under push button start. I'm designing a system for my '98 and hope to have it done by May. Since the 1998 has a passive FOB it can operate similar to modern cars, I'm using a Lexus button instead of the three button GM type. This means that the door will unlock when approaching the car and then allow the ACC/RUN and START to function by pushing the brake pedal while pushing the start button. Not pushing the brake pedal allows ACC and RUN functions to be commanded with the push button. The resistor in the key is easy to duplicate as is the key in/ out of the lock switching. Later C5's without the passive feature would require unlocking the door by pressing the FOB unlock button but everything else is the same.