Thanks Scott!Assuming things are the same on a car without auto-pulldown the ground for the trunk popper is through the mounting of the popper, part attached to the trunk lid, as I recall. There is a single power wire running to the glove box yellow button. The button is a normally-open switch that closes when you push it and a single wire goes to the solenoid on the trunk popper. I believe the factory wiring only allowed the popper to function with the key in to better secure the trunk if someone broke into your car. When I added it to a car I used a source which was hot all of the time.
Power will find a ground in strange ways if the ground is bad. Maybe things are on the same circuit and there's a bad ground which is causing the power to take an alternative path to find ground.
Also be careful going solely by a multimeter as you have no load when you check voltage. If it's a bad ground you may want to also have an "old school" test light with an incandescent bulb as your load.