this would make the lights on when the door is closed. if you are wanting these on when the door OPENS, change 86 to 12v.

when are you wanting these lights to be on?

Lights on when door is open.

I'm doing my best to try to figure this out with the help of Dr. Google. Unfortunately, I don't understand the 12V (constant?) connection. Can you also note what the other connections should be? One wire is hot all the time. The other one is only hot when the door is closed..

85 door wire that changes
86 12v battery constant
87a battery constant
30 led positive

honestly if youre just using a couple of small leds you can just tie the led negative to the door wire and the positive to 12v. if you wanted the added illumination part you'd need some diodes.

i could make you a harness but it likely wouldn't be cost effective by the time i bought parts and charged minimal labor.

All relays use that same pinout numbering. So you just want any automotive relay, lowest amp rating you can find, since no real power needed here, and they'll be smaller. So any aftermarket relay for stuff like light bar,etc. Except small, cheap.

A relay is an electromagnet, that pulls a switch when energized. Two of the leads get switched ON when magnet pulls switch, other two leads turn OFF when magnet pulls switch.

The leads that turn ON are called Normally Open (bc they close when magnet pulls switch), the ones that turn OFF are called Normally Closed (bc they open when magnet pulls).

You'll see these called NO and NC.

So you want the door light power to make the magnet pull the switch. So one lead of magnet goes to ground, other lead of magnet to door light. This will pull magnet switch with door light power. Door opens, power to door lights, power to magnet, magnet makes switch switch.

So far so good?

Now, you want the two N.O. leads to act like a plain ol switch wired inline with your lights.

what i said relies on where you grab the door circuit btw. i'm going off the door switch which is a negative trigger. the dome light always has power, its the ground that switches.

I've been reading between normally open and normally closed. @Car Analogy, you did a nice job explaining the difference between the two. I really enjoy the learning process of fiddling with things like this.

I think I need a relay that will act as a normally closed manner. That said, my somewhat educated guess is the leads get attached to the relay as follows:

85 = door wire that changes (green yellow wire, which is energized when the door is closed)
86 = ground
87a = LED positive
30 = 12V always on constant (white blue wire)

I measured the voltage yesterday of the green yellow wire when the door is closed and open. Closed it reads 12V. Open it read 2V. There seems to be a little voltage there still. I don't remember that the last time I measured it.

All generic relays have a NO and NC pair (they share one common lead).

In this case, wiring relay magnet wires to ground and to door wire will result in a relay that always has power applied when door closed. Its always pulling on the switch.

So if you then use the NC relay wires to connect your new lights to power and ground, then when relay magnet is engaged with power, your new lights will be off.

When door opens,power to door wire goes away, relay will go back to default positions, so NC wires will turn your new lights on.

The things to be careful with here are:

You don't want your new lights on all the time when car parked. So you want to power them using power that is only on when car is on. So use the accessory power tap on fuse panel under dash. Use an inline fuse.

Also, does the door wire with 12v when door closed always have power, even when car is off, parked? If so, relay magnet will always use power. Not much, but a constant draw. Battery will go dead a little sooner when car sits for a long time. So trickle charger needed a little sooner. But really anytime car will sit for several weeks it should be on a maintenance charger. That isn't changing.

Since you're using power for lights that is only on when key is on, it doesn't matter relay is on or not. Lights will be off either way.

Why would you want to have a relay resting in the energized state? Just use 12v on one side of the coil and the door trigger on the other. Open door, relay triggers. Then there's no current draw to even worry about ever.

I'd like to thank everyone for their posts and helping my understand how I could use a relay to accomplish my objectives.

I wound up throwing in the towel and tapping the dome light directly. I typically prefer not to cut or tap wires to do things. I was able to use small posi tap connectors to tap into two wires. I followed the existing dome light harness that runs towards the passenger side footwell area and tied everything together under the dash.

Functionality wise, it actually turned out to be a good outcome. The footwell lights will come on whenever I move the sliding switch to the right to turn on both dome lights and also when I turn on the passenger side dome light with the push button switch. I think I could have done the same with the driver side push button switch, but since there was less room to tap the needed wire on the driver's side push button switch I decided to just call it good.

I would still be interested If any one does figure the magic / schematic of the Honda footewell lights.