Switch/relay for a Tonneau cover bed ligh

I want to put a switch of some sort on my tonneau cover in my truck so that it knows when its open/closed (or should I use a mercury switch?) Anyways how would I wire it so it has dual functions 1) being trunk pin switch for the alarm 2) tells something (relay?) to turn a light of some sort on that I will put in the truck bed.04 Silverado EC

What i did in this situation was go to walmart any buy one of those ugly push button start switches.  There are in the automotive section.  I got the box looking one.  Now it is connected when pushed switch, but the box easily comes apart and the insides can be reversed so that the switch is essentially a momentary disconect.  Depending on you style of bedcover you will have to make a mounitng bracket.  I just used some l angle aluminum and bolted it to the side of the bed and made sure the bed cover presses the switch in fully when closed.  Add a nice light and your done.  There are some better ways to do this but mine is very cheap and the walmart switch is almost indestructible.  Mine had been like this for 2 years now with no problems what so ever.