Rear Defogger Fuse Issue

[Boris3]

Winter projects time and on the list are a couple of small issues with the car. Been searching the forums on a wiring/fuse question but no luck, so I figured I'd post the issue.

Every time I turn on the rear defogger the 30 amp #8 fuse blows immediately. If the headlights are on, the defog switch also goes dark (#23 fuse is fine). If the aftermarket head unit is on when I hit the defog switch, the head unit also goes dead. If the head unit is off when I hit the defog switch, the #8 fuse still blows and I get no power when I try to turn the unit on. Replace the #8 fuse and the defog switch light and radio work fine. Hit the defog switch again and I'm back to square one.

I expect this all may be related to a PO's creative wiring on the head unit install but on the off chance that this is not the case and I'm dealing with multiple issues and before I start taking things apart I was wondering if there are any common shorts in the rear defogger wiring path that would lead to the #8 fuse blowing?

[jlevie]

I can't see where there should be any connection between the audio gear and the rear defroster. It sounds to me like you need to remove the audio gear, revert the car's wiring back to factory configuration, and then re-install the audio gear correctly. There could be a fault in rear defroster circuit and you can check that out after the wiring issues have been fixed.

[Jaxx_]

I'm gonna guess that the PO wired up the rear fogger as the 12V feed for the head unit. It's an ACC, isn't it?

From that the draw between the defroster and head unit blow the fuse. I think your diagnosis is correct. The fact that the fuse isn't blowing when the head unit is off pretty much says that there is no short. unless of course, the PO decided to make something like the defogger's 12V a ground or something crazy. Never, ever trust your PO.

[Boris3]

Thanks for the replies. I'll open things up and see what sort of installation spaghetti I find.

Head Unit is a Dual XDM6820. Last time the fuse blew though the unit was off. I had forgotten about the issue and turned on the rear defog switch to melt some frost off the window and the fuse blew.