My car is hibernating right now but after working on it all last summer and fall, I have a question that I have not been able to find an answer for. In that mess of wires under the dash , I was checking the wiring plug to the control switch because my heater was totally dead. I found another unused multi wire plug in there. when I unplugged one and plugged in the unused plug just about every thing started working. also I replaced the Piano shape ECU but I have seen some ECUs on line marked convertible which mine doesn't have. Two questions !. Why is there a second plug for the heater control
2. How is the the convertible ECU different (MK3 system)
Thanks in advance for advice