Hello, As far as I know there are 2 trunk hinge tension rods that hold that open. They are made of spring steel. They have bends to assist opening and hold. Maybe yours are broken or wore out. I'm sure someone has a pair on this site for sale. I have a pair if interested I can look for them. Also maybe they came out of the area where they are mounted at.
That is one heavy trunk lid
Sure you will be glad to correct that issue soon
Sure the answer will be along shortly
Trunk rods are different for different models. Be sure to get the correct trunk rod set.
Look in your 1963 Cadillac Shop Manual Illustration Fig 16-78 on Page 16-69, then Rear Compartment Torque Rod Adjustment on the next page 16-70 section 80. Rear Compartment Torque Rod Adjustment.
Note that you have 3 positions to adjust the torque rods. The lowest position 1 gives least upward force and the highest position 3 gives most upward force.
If torque rods are in the lower positions move up a notch and try the trunk lid. If all ready at the top #3 position torque rods are probably weak and worn out.
I have a good stock spanning most if not all body styles for $25 each or $45 for a pair of you need them. If you end up needing any email me at email@example.com
Here is the illustration you will find in the shop manual: