Okay, '96 Z28 convertible is being built and the mechanic found an odd plug in the wiring harness. It's located at the rear of the engine and it is 4 pin "oooo". What is that for?

He also found a flexible tube--looks like a vacuum tube--way back in the corner near the clutch master and disappearing into the firewall. What is it?

 

Here's a 95 V8 wiring harness, courtesy of Shoebox:

http://shbox.com/1/harness.htm

The O2 sensors connectors are 4-pins, flat.

 

Thanks! That looks like it, but it's peeking up from behind the intake. Maybe it's twisted or something I'll let them know.

Any ideas about the vacuum looking tube?

 

Sometimes people confuse the cruise cable housing for a vacuum hose. It loops back toward the firewall, then low and forward toward the cruise control that is mounted low by the LH inner frame rail.

 

Aha! That sounds like it may be it. Thanks.