2005 Super V8 with the four zone climate. Right now I have poor heat up front despite new heater core. Daah vents put out cold air.

Rear blower isn't working at all. The display works, the buttons work. However there is no movement of air.

Where do I start? Fuse? Relay?

 

Ok, did you properly purge cooling system? Forum search.

Check climate settings within navigation screen and make sure the rear climate control is not "locked out" to the front part of system, meaning the fan, temp settings can only be adjusted via front panel. (Not near my CAR now but that should get you started)

 

Yeah it was purged earlier this year. It sat at the dealer for a year and they supposedly fixed this issue.

I manually adjusted the rear and unlocked the controls. Unfortunately the blower doisn't come on at all.

 

Checked the fuse in the boot fuse box and it's good. Hmm

 

The four-zone climate control system has two fans, one front, one rear, and both have a fan speed control module with cooling fins protruding into the air stream. These modules are specifically stated in the technical notes as not being interchangeable. However it is a well-known fault for the front fan module to fail, (it happened on my previous X350). Later modules have longer fins and last longer, so obviously this was the problem - they overheated ! Four zone systems are far less common, but it is likely the rear fan control module is of similar construction even if not identical so would suffer similar failures. So fan not working is likely to be the speed control module has failed.

As for the cold air at the front, (and probably at the back when the fan is fixed), the V8s have a separate coolant pump to get hot coolant into the climate control heater matrix. If this has failed the heater won't be getting enough hot coolant to heat the cabin.