Originally Posted by willwren
Don is on the money here, too. Other than simply replacing parts one by one, there really isn't a good way to do this. We don't simply mean pulling codes, either. We're talking about a 'drivealong' to verify all the individual components. Even your TPS is a suspect at this point. Too many variables.
Replacing the switch is a major investment in time and effort to do just on the theory that it might fix the problem. At minimum I think you'd want to check for the correct voltages, etc., at the underdash connector where the harness going up the steering column to the switch plugs in.
Having said that, I have had personal experience with the thin-gauge wires to the switch breaking inside their insulation at the point where they jump the gap from steering column to turn-signal lever, from flexing when signalling turns over the years. However, this was easy to diagnose since the system did still work most of the time, but would fall out of cruise mode if you wiggled the turn-signal lever while driving. If yours is completely DOA right now, I wouldn't go pulling the steering column apart in the absence of any clear proof that that'* where the problem is.
I'm sure the manual has one or more problem-diagnosis trees on this problem, and it'll really help you to locate a copy.