This is how I would do it as well.Back in the day I connected the lights through the normally closed side of a 5-pin relay with the ground supplied by the existing door switch.
Older crown vics used a mercury switch in the trunk, then an electronic one, used to hook into them to run flashers when the trunk lid was opened.Is there a benefit to the 5 pin over a mercury switch?
This is exactly what I'm after as I don't have a trigger to tie into.Mercury switches are more useful for camper beds with the fold up glass or something like that, where there's not a signal you can put into a relay