The engine coolant lines leading to the rear heating unit failed and anti-freeze dripped out. The solid aluminum lines cost over $1K for the part alone, then add labour on top. I opted for the cheap fix which was to crimp the lines off near the engine and for go rear heat ($300). My GMC dealer told me that the design was poor. Insufficient fluid movement contributed to sludge building up. I am not sure how that contributed to Al corrosion, but I accepted that they saw a higher number of these problems showing up. If I used a local mechanic, I may have been able to solder Al (specific skill) or replace a section with tubing and clamps. When I thought about the inconvenience of a leak in the middle of nowhere vs no rear heat and getting it fixed that day, I went for the by-pass solution. No leaks, but no rear heat (other than recirculating the front heat through the rear blower which still works.)