trunk light relay

How do I hook up a relay for my trunk light to convert the (+) signal to a (-) signal, is there an easier way to do this than with a relay. Thanks alot

Relays are your best bet.  check it  out!

Thanks for the diagram, now I am really gonna newb ya out.  What do the three black lines represent coming from the 30 and 85.   86 is the wire coming from my trunk light.  Is 87 a whole new wire with neg output or is it just the other end of the light wire that I had to cut? THANKS ALOT.  I hope modifing that diagram wasnt copyright infringment, oops.