I want to hardwire a Valentine One in my 2003 Prius

The Prius can do 80 or 90 miles an hour? Sure it can, so why not have a Valentine with you?

I'm not so sure if you have a 12v IGN near the light module, but you can test it with a wire tester.

Why do you want to connect your V1 inside the engine compartment? I connected mine to the IGN wire coming out of the IGN switch and ran the wire all the way to the sun visor.

 

If you have the extended hot connection hard wire, and you can remove the dashboard panel under the steering wheel and the ignition cover. All you need is a Philips screw driver and a 10m socket with a ratchet, and you can follow the ignition harness. Look for the black/yellow (+) wire; this is the ignition wire. The V1 will power after the car is on or running. Then lead the wire all the way to the top of the windshield.

 

If you don't want to go digging for an IG-switched circuit but there is a constantly-powered circuit nearby (as for the map light), another option is a voltage-sensitive switch. These are sometimes sold prebuilt as "battery protectors" that shut off below 11 volts or so, with the idea that you'll hook up some load you want to run even with the ignition off, but shut off before the battery is really drained.

The same idea, only built with a higher voltage threshold, makes a reliable Prius READY-ON detector, because a fully charged lead-acid battery is 12.72 V but the Prius converter puts out 13.8 in READY. So if you set the threshold between those values you can have something powered when you're in READY and off when you're not.

I used the idea here: that was physically large because I wanted an expandable multicircuit fused distribution thingy for the trunk. If all you want is a single circuit for a small detector, you shouldn't need much space for the necessary components.

-Chap