If the '70 switch is anything like the '71 which I suppose it is, it is activated by a mercury float switch imbedded into the socket body of the switch assembly. This is not serviceable. The top of socket may just need cleaning and lubrication. Try a round brass bristle brush and some penetrating lube to get the spring to rebound. If the spring is broken, you maybe looking at a new trunk light switch. They are affordable on eBay.com and are not impossible to find with an unmolested wire harness.