And this is going on your 53 Ford?

Does your truck already have a third brake light? If so, just catch the wire
to it and extend it to the cap's light. If not, you have to get it at the
break light switch as it splits to the individual brake lights by the turn
signal switch. You decide where you want the power for the dome light to
come from depending on how you want it to work.  Are you going to put a
switch in the cab to turn it off? You can actually get it from the other
side of the brake lamp switch, that way you can make both connections in the
same area and it will be easy to remove if you want it out. Be sure to get
some inline fuses and install them in any wire you add.

Al