turning on map lights with door how?

Car is an 02 Subaru Liberty (Legacy)

I've lost my roof light with the install of roof LCD, it has a roof light but only lights the rear passenger area. I'd like to have the 2 front map lights come on with the door as well. Both map lights are fed by constant 12v and a push button for each light applies an earth.

The door open signal (an earth) comes from the Keyless Entry Control Module. My plan is to add a 3rd switch to the map light cct, fed by the door open earth, with the output of the switch going to each light. but, I don't want both lights coming on when one is manually turned on. I could simply run each of the 2 output wires from the new switch through a diode to the lights, but I'm not sure about any problems with running light power directly thru the diodes.

Can anyone offer suggestions ?

Thanks

Well, it turns out the factory wiring diagrams aren't to be trusted.

It shows constant 12v Blue Red (BR) wire going directly to the map lights and black wire going via the map light switches. When i removed the map light assy I noted that the wire going directly to the light is grounded to the car using the mounting screws. Upon inspecting the connections further I see that the BR wire is actually fed to the map light switches and the earth is tied directly do the bulbs. Opposite to what the factory diagrams indicate.

Well throws my plans a little. I was going to run an extra switch in parallel with original switches, all switches supplying an earth. Now I'm stuck where I need to switch earth to the same side of the light that has 12v switched to it.

Easiest way might be to swap the light wiring over to match factory diagram and then remove the link (or use an insulation washer)  that grounds the unswitched side of the bulb directly to the car body when the mounting screws are fitted.

Still leaves me with my Diode query.