The speedo problem is most likley a bad instrument cluster. As for the rough idle, the heater should not affect on the idle quality. If it is in defrost mode, the A/C compressor will cycle. This will have an effect on idle quality.
The first thing to do is have it scanned for trouble codes. Use a scan tool to see if there are any cylinders misfiring.
Friday, March 26th, 2010 AT 8:25 PM