The sail switch is found on all furnaces with air handlers. Look for a switch in an open area past the blower with a metal tab looking like a sail. It is a safety switch. The blowing air blows the switch open signalling the furnace that the blower is working. If the blower doesn't work, then the burner could cause the furnace to overheat.
You probably need to have an HVAC person look at it who can test everything. You've already done what it seems like auto repairmen do and thats just replace parts till its fixed without really knowing how to fix it.