I have a freeware plane that has an interior cockpit light that comes on when you select key L as do all the other lights, interior and exterior. My question is there any way to make the main interior light turn off with the other lights on?

If the gauge is created in C then without creating a new gauge, no. If the gauge is created in XML it may be possible but you'll need an answer from someone else on that score. Which plane is it? If I know that then I can take a look and give you a more definate answer.

Check to see if there is a cabin light or panel light switch in the virtual cockpit.  Most freeware aircraft use a simple cabin light for cockpit lighting.

The plane did not come with a cockpit light or switch so I installed a Shockwave (A2A) interior light. The problem is there is no switch for this light so it  only comes on when use the L key and that also turns on all the outside lights. Not sure if the Shockwave light is XML or not. 

 

It is just a light added to the panel vc config so would there be a config command I could use?

L is a default key that toggles ALL lights on/off.

The plane did not come with a cockpit light or switch so I installed a Shockwave (A2A) interior light.

If you placed the "light" via the aircraft.cfg file, then by setting the light to use the "cockpit" lights circuit (type 4), you can then use the default Shift-L keycombo to toggle them on and off:

[LIGHTS]
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing 
//       6=taxi, 7=recognition, 8=wing, 9=logo 10=cabin
light.0 = 4,  4.91,   0.00,  1.64, fx_vclight,
 // use Shift-L to turn on/off

Thanks, great advice and it works perfectly.

Wow! You must have watched me write that post...