Interior lights and tach issue 69 dart GTS

[chasb]

I have a vintage SUN 9000 rpm tach . It was working beautifully until last week. Then I noticed at night that the tach and its light were not working if I had the interiror dash lights on, but if I turned the interior lights off the tach starts to work again but with no tach light??

I hve an MSD ignition in there as well

Any suggestions?

 

[67Dart273]

You.....have......a......ground......problem