['68OrangeSunshine]

Since AC Delco went to Mexico, I don't trust their quality control.
I now use Denso spark plugs for that reason. I hear they're made in Battle Creek, MI.

I have had a defective 'new' boot-tap Hi-Lo switch, but it was a cheap Checker Auto ''house brand'', not an AC Delco.

The issue I found on my '71 Jimmy, was all 4 elements on all 4 bulbs burned out. This was caused by a runaway EVR [external voltage regulator]. When I replaced the EVR, the front engine wiring harness, and the bulbs, and grounded the Lights harness at the passenger side radiator wall under the Batt Box, [where the PO had hidden a cold splice - no solder, no crimp, just twisted wires and e-tape ]
it has all worked great ever since.
Sylvania replaced all my halogen lights under warranty, but their engineering section required me to submit milliAmmeter readings on each lamp. Had to build a special harness and buy a surplus Ammeter to get the readings. But they delivered, after all that.