Believe it or not i had the same trouble fitting my led scanner but i had trouble finding a live.
Its different in a Ford you can take things apart and pick up live anywhere.It must be just me but i am scared to do the same on my merc.
I did it in the end but i am too emmbarressed to say how.Where did you get yours?
If it works like the old alarm LED,s I used to install, you don't use "ground" at all.
This is from a dim memory but try this....
Find a constant hot wire, maybe cigar lighter, lamp circuit, etc. - This is your LED power source or "+" wire.
Next find a wire that is only hot when the key in on, maybe glovebox light, mirror control, etc. - This is your LED Ground or "-" wire.
Before you solder, crimp, tape or do anything permanent, check the LED operation with Key off and on and then Key off and on with lights off and on.
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