You need to bite the bullet and run a continuous wire from the trunk light to the LH side of the instrument panel. That end can pick up fused, constant hot power from a green-yellow tracer circuit used for courtesy lights. To do this requires a male bullet and female bullet connector to make a "Y" jumper. Picture at the top of the "Y", a male bullet on one leg, and a female bullet in the other leg (the base of the "Y" goes to the trunk). The jumper part is about two inches long. It allows you to unplug the existing green-yellow lead, plug in your new lead and reconnect the green-yellow lead to your jumper. This "Y" jumper technique is used on the Rally Pac and other light circuits. The easiest point to get this power is the green-yellow feed for the console. You can also get this circuit at the courtesy light switch at the door. The bullet connectors you need are a 0.187 inch dia type, a Ford standard. Two female, one male are required. Removal of the LH door sill, LH rear quarter panel and LH front quarter panel are required to properly route the wire. I strongly recommend you hook up the trunk light FIRST before finding power. That method keeps the fuses "intact".
Resist splicing. (I do not believe in splices, and I avoid them.)
Jim