It turned out to be the cruise control/idle speed control unit.

Part number: 1245455532.

My mechanic had told me he thought it was the wiring harness, which I bought independently and had him inistall it. 'Course that wasn't the problem! I'll try to sell the old one on ebay.

The mechanic at the dealership that diagnosed the actual problem said that the unit (brains?) looked as though it had been fried. They'd never seen that happen before.

__________________

Current:
2014 VW Tiguan SEL 4Motion 43,000 miles.

2016 Hyundai Santa Fe Sport (wife's).

Past:
2006 Jetta TDI 135,970 miles. Sold Nov. '13.
1995 E-320 Special Edition. 220,200 miles. Sold Sept. '07.
1987 190-E 16 valve. 153,000 miles. Sold Feb. '06.
1980 300-D 225,000 miles. Donated to the National Kidney Foundation.
1980 240-D manual, 297,500 miles. Totaled by inattentive driver.