You might try some WD40 sprayed into the latch mechanism (up in the trunk lid) to flush out any existing water that may have gotten in there, and to preclude further water entry, that may be freezing and fouling up the latch.
2005 300C BRILLIANT BLACK, DRL, Mesh Grill/Vents, Chrome Pillars & Fender Trim, MDS Indicator, Quad Brake Lights, Tinted Windows, De-Molded, K&N Aircharger, 180 T-Stat, Flashing Sidemarkers, Predator Cust. Perf. 93 CAI Tune
1997 MUSTANG COBRA 'VERT, TRIPLE BLACK, Vortech S/C'd., ECT Mod., 4.10 Rear, 18" Saleen Wheels
1950 FORD TUDOR FLATHEAD V8, FLAT BLACK, 54 Chevy Grill, Nosed/Decked, Frenched Headlights, Air Shocks, Three-On-The-Floor w/OD, Dual Glasspack Exhaust, Dual Two-Barrel Carbs