Ok, I think I've found one possible culprit. Hidden under the dash is a push button that sends power to the glow plugs. This obviously wasn't stock.

When the button is held in, there's 12V directly to the glow plug wiring strap. When it's not held in, there's no power.

I'm wondering if there's an easy way to "jury rig" the glow plugs to work properly. Anyone know? Thanks.