Finally got this fixed. I decided to pay someone rather than doing it myself. The autoelectrician was not impressed. Apparently the bend in the wiring loom as it goes through the firewall is too tight and the loom was pinched when the heater was installed exacerbating the issue. If that is typical then a lot of people have similar issues to look forward to as their mileage increase.
The heater and the ECU had to be removed to get enough play in the loom to examine it thoroughly and fix it, so it's not a small job. He observed that there is plenty of space for a different routing that would not have had any issues.
