99 Grand Marq: Trunk not "popping-open", anymore

Hello:
I have a 1999, Mercury Grand Marquis LS, 4.6L, V8 upon-which I recently installed a very conservative-looking black, metal border ("frame") around the rear license plate. Before the installation, when I'd open the trunk, either by pressing the "Trunk Open" button in the cab or, by remote, the trunk-lid would "spring" open; all the way. Now however, it'll usually "pop" open about an inch and a half if it's warm outside... but if it's cold, it may not "pop" at all-- and I'll have to use the remote, while pulling-up on the lid.
Presumably (obviously), this issue is being caused by the added weight of the plate/border, which IS particularly thick&sturdy-- as plate borders go(!)

So, I come to ye' men (& women) of great knowledge, to ask if there's an alternative to my having to choose between "springy-trunk lid" & "sleek-looking lic. plate border/frame"!

I seem to recall hearing (somewhere[???]) that the trunks' spring-tension could be adjusted:
1) Is that true?;
2) Would/could that be a cure for my plight?;
3) How might one go-about doing that (backyard mechanic kinda' project, or one to take to the "shop")?
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The spring tension can be adjusted. It can be somewhat dangerous if the spring slips while adjusting it. You may be able to go to a body shop and for a minimal fee, they can adjust it for you.

Before you do anything else. Clean the trunk gasket rubber. Mine was doing the same thing and I found that the trunk lid was sticking to the rubber gasket. I just wiped mine down with clear water and put a light coat of protectant on it (black magic) and its fine now.