1985 Other Subaru Models Interior Light Tailgate Switch

I have just bought a Subaru 1985 GL Staion Wagon.
The tailgate switch does not appear to work properly as the internal lights[both of them] do not turn off on shutting the tailgate. If I force the tailgate down hard it sometimes works.I have taken the catch/lock from the tailgate but cannot get to theswitch inside, is there any way to get into the catch and check it out, or is there another way of adjusting it?