A couple tips:
1) Don't use one long piece of tape since it has a tendency to shrivel just like yours did.
Add relief cuts all along the length of the spoiler and use shorter pieces of tape.
2) Use some heat when applying and also keep it taped down for a while with painters tape.
It's hard to get it sitting totally flush and flat so there's always a force pulling up against the tape so if it's not fully adhered it will pull the edges off.
If that doesn't work try to rough up the surface of the spoiler to create some ridges for the tape to bite into and maybe even use an adhesion promoter. The tape will stick to your trunk like glue, you just need to make sure the tape can properly adhere to the surface of the spoiler.
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