This past weekend I did the dreaded sparkplug and plugwire change at 88K miles.
I also replaced the battery ground lead (negative post to body) because the post clamp
on the original lead had become so deformed that it could not be properly tightend
to the post anymore. All went well until I noticed last night that when I noticed
that turning on the headlights made the radio display go dark, like it had changed
polarity, only the display characters were mostly ureadable. (This is the OEM Nav radio.)
I also noticed that when the headlights are turned on - the little red light
in the HVAC switch dims slightly. The instrument cluster lights are fine and
the rheostat works 4.0.
There are no other unusual symptoms that I've noticed. Has anyone on the Forum had
a similar experience? Any insight would be appreciated.

 

The NAV radio display going negative is normal. About the lights dimming, when you turn on your headlights, it draws a LOT of current and lights will dim. I would not worry about this unless aomething else electrical goes haywire. Drive her and have fun!

 

Inspect the connection at the chassis end of the new cable. When you replaced it, there may have been a little oxidation on the sheet metal and this can cause havoc with our e-systems! Sand/Scotchbrite the area before screwing it together.
It does not need to have visible rust present to be an issue.

Perhaps the reason the NEG battery terminal end of the cable was damaged was from mistaking the problem area? Just askin'.

 

Thanks for your input guys. I figured it out and fixed it this evening. The radio display has three settings - Positive, Negative, and Auto. It turns out that having the battery disconnected reset the radio to default Auto setting. With Auto setting, the radio display is Positive when the headlights are off, and Negative when the lights are on. Fixed it by resetting the display to Positive. Viola!

 

Exactly! Just like post 2.