The interior light of Mk3 or later Classic Mins should turn on when either door is opened. The control of this function is by way of a mechanical switch mounted in the door jam. Originally for MPi (multi-point injection) Minis from 1996 when the wiring was changed from bullet type to spade connection. This is the switch that fitted into the A post to turn the interior light on and off, the earlier switch is no longer available so if you have an earlier car you will have to change the bullet connector to a spade connector.