I wired my retrofitted front fog lights direct to the battery, switched with a relay off the rear foglight switch and never had an issue. That's to a 2008. I find it a bit hard to believe that the Tig would know the "total" draw from the battery at any one time as there are a number of circuits that are not load monitored as far as I know. That said I'm no auto-electrician !!!
Cheers
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