led lights w/courtesy lights and switch

I am installing footwell lights and underseat lights. My issue is that I want the lights to come on with both the courtesy lights and be on a switch which works when the ignition is on. I have heard a relay is probably going to my option here. I just don't know how to make this work. Any help is appreciated.

Cap

Oops sorry forgot this bit,I have a Mazda 3 2008.5.

The ignition switch is easy.
So too the courtesy lights is they are hot switched - ie, like the ignition, +12V switching.
The you merely connect each via a diode to a relay that powers the new lights.

But if the courtesy lights are ground switched, it's a bit trickier.

Can you describe the courtesy light wiring (ground switched, or ground switching a hot courtesy light extender/dimmer, etc)?

Otherwise I'll have to find the right wiring diagram....

The wiring is similar to the Euro spec Focus (what a surprise). If you go to the dome light ground switch feed that will give you ground switching, if you diode the dome light feed before the join, then 12v+ign to switch, switch to relay 86, 30 and 85 to a good ground and 87 to aftermarket lights you will have what you need.  Leave the diode out and the dome light will also come on. Wire the aftermarket lights 12v+ constant and ground switch.

Thanks to both of you guys. I should be able to get it from that right there. I think I have learned more about relays since I thought about doing this that I ever wanted to know. Your right about the Euro spec Focus, the Mazda 3 was the test bed for it and the Volvo S-40. Same chassis by volvo and some internals with Mazda doing engines. Very very helpful and hopefully I can pass this info on and I'll let you all know when I succeed.

Ok, couple more questions. In this setup would this create a constant on condition for the relay and draw current from the battery continuously? I know relay draw is relatively small but don't want to take chances if I don't have to. Other question is which way should the band of the diode face? Towards the source of power or away? Thanks again,

Howie's setup will only energize the relay when the ignition is on along with your added switch, so there's no problem with the relay draw when the car's off-

If you want to add the diode to the tap from your courtesy light, the band of the diode (cathode) should face towards the courtesy lights.
Note this setup will require the diode to pass the current needed for the aftermarket lights, so they should be less than 1A for the run of the mill 1N4001 diodes. If they need more current than that, you can switch the connections around so that the relay is ground switched instead of (+) switched.

You can do that by having the relay coil terminal 86 connected to your ign switched 12V and your added switch connected between relay coil 85 and ground.
Then, wire in the diode between your courtesy light gnd switch and the relay coil term 85, again with the band towards the courtesy light side. This way the diode will only need to pass the current for the relay coil, which is small.

Hold that thought- that won't do for what you want, since they won't come on with the ignition off.
You could just use a second relay for switching the aftermarket lights from the courtesy lights, but there may be a better way if you want to save the expense of another relay; have to think about it-
Since you're using LEDs then your draw is probably less than an amp, so Howie's original setup should be fine.

Okay, here's a way to set it up- let's use the 87A normally closed contact of the relay.

Relay connections:
87A to courtesy light switch
87 to ground
30 to ground switch of your aftermarket lights
86 to ignition switched 12V
85 to your switch

Switch connections:
relay coil 85
ground

Aftermarket light connections:
+12V constant
relay coil 30

That should be another way to do what you need-