You can use a multimeter to check continuity and/or resistance, or voltage stopping on the circuit path to the bulbs when they should be one, as well as checking each individual bulb for being open circuit (blown).
Considering the age of the vehicle, once you have it apart I would go ahead and put new bulbs in it anyway.
If it is the ribbon connector, rarely I find the contacts on the film can get a corrosion or dirt buildup and just need rubbed with a slight abrasive cloth like a dry paper towel, or with contact cleaner on it if there was an especially dirty operating environment. Contact cleaner might also be used in the ribbon connectors or if they too are quite bad, a thin piece of single ply cardboard, like a cereal box wall, can be cut to size and slipped in and out of the connector, also with contact cleaner added if needed.
Before all that, do use a multimeter to find where the circuit is broken.
