2000 Pontiac Grand AM GT, recently would not start, ruled out dead battery based on interior lights coming on, when engine turned over the trunk would open instead.l am thinking it is a wiring issue?Is this expensive to fix?

just because the lights work does not mean the battery is good use a meter or bring battery to parts store to have tested a bad battery can make some odd things happen

I'd go buy a good battery charger & tester & take the battery out of the car , put it on charge & be sure to read the instruction manual before using the charger , plus places like walmart sell very cheap chargers & if u want to u can simply return it if u find it does not help.Plus u can simply take the battery to a sales outlet that sells those brands of batteries that offers a free battery check.
It does sound like the battery is faulty or the alternator is not charging the battery , plus the main comp=ECM will do funny things when the battery is low like opening ur trunk etc.

Hope that helps & best of luck.By the way check the battery terminals to see if their loose or rusty or corroded & if so tighten them or clean them with a small wire brush that u can buy in most dollar stores now adays.

cheack your battery termanl