Stop replacing parts and do some diagnosis. Disconnect the connector to the cruise control and measure voltages on different pins. You have to make sure voltages are present for power, ground and signals from the switch.
Here's the link for the diagram of the circuit:
http://rapidshare.de/files/14455130/98_svt_cruise.pdf.html
If you can't read a wiring diagram and don't know what pins to test, me or some other electrical guy can help.
BTW, it could be as simple as a dirty switch in the steering wheel. A friend had a 2001 Saturn with a non-working cruise. The supposed "techs" at the dealer diagnosed it as a bad cruise module ($500+). It was only a dirty switch in the steering wheel (free fix)!