1996 Mercury Sable trunk won't open, any suggestions?

Well my trunk doesn't open most of the time. It only opens at night when it is cool. I bought the car used a couple years ago for $7000.00 and didn't receive a key for the trunk so I only have the little device on my key ring. I have been told that the thing that unlatches the trunk needs to be replaced and I need to have a new key made costing me about $200 or more. Is there any other way of fixing this or no?