2008 e91 Touring rear window demister

[jrpat]

The rear window demister on my 2008 E91 Touring doesn't work. Things I have checked:
Fuse - all good
Relay - Operates, and I've checked continuity of the relay contacts - all good
Element continuity - Checked continuity of the window at the connection box at the top of the widow. Shows what looks to be an appropriate resistance.
Volts - Unplugged the demister in the sealed box at the top of the window. With the motor running, and the demister turned on, it shows good voltage.

Here is where things get odd. Having just checked the voltage on the connector as above, when I connect the window I suddenly get a reading of 0V across the circuit at the same point. Disconnecting the demister does not cause the voltage to come back up (Fuse and relay still okay). However, if I shut the car down, and restart I have my volts back again.
Checking faults on my computer do not indicate any battery issues.
Yes, I am aware of temperature and timing issues as they relate to the demister, and none of those appear to be a part of the issue, but I am stumped at what is causing the circuit to shut down. Any thoughts? Thanks, J

[relative4]

Disconnect the IBS from the DME connector.

[jrpat]

Disconnect the IBS from the DME connector.

Just tried that. Sadly, no change . . . . apart for the Airbag and Seat belt warning lights coming on, and not going off when everything was reconnected.

[relative4]

Disconnecting the IBS will not cause those lights directly but could trigger such things if the battery is bad. Have you scanned with ISTA? What is the age and condition of your battery?

[jrpat]

Haven't yet scanned with ITSA, but suspect that you are right about a bad battery - that would explain the issues I'm seeing with the demister.
As for as the Airbag and Seat belt warnings, I reseated the IBS connector and cleared the fault. All is good.