My center backpad is not cut for a light so I'm adding an led light under the dash.

I think i hook up the pos. to constant power but how do i setup negative so the light goes on with only one door

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You *should* have the wiring floating around the car for that. In the event you don't, just run each negative to the door switch and then "double up" the wires to the negative side of the light.

What light are you running? I was thinking of doing the same.

-Aaron

Just added a door activated light to illuminate the ignition switch in my 911 last week. Most 12 volt LEDs have dedicated positive and negative wires. If wired backwards then no light. All you need to do is run a jumper wire to the wire coming off the door switch. Since it is a ground wire it is brown. Run another wire to a constant power wire, which is red. (Not red with a white line which is switched on power). You could run an in-line fuse but because LEDs draw so few AMPs it may not be necessary. The only downside is that there is no one way to turn the light off when the door is open unless you wire in a switch which probably isn’t necessary.

I am planning on some led strip lights under the dash. I have them in my gauges and they are great.

There are two negative wires of the door switches. If I connect them to the same neg side of the light then both doors would need to be open to cancel out the current.
If one door is open the circuit is still complete and the light is off.

Do I need two lights?

Using one of these stuck underneath the bottom of the dash.

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You don't need "both" to doors to be open. You only need one path to ground. Try it for yourself with a volt/ohm meter.

Funny. Just looked at that one, but I'm going to try some inexpensive, stick on, flexible, 12V LED's.

https://www.amazon.com/gp/product/B01M2U65R...=UTF8&psc=1