Theres a door pin switch on each door. The switch is NO (normally open) when the door is closed. When the door is open the switch connects to ground.
You can either use the door pin switch or the door trigger wire on the car. The difference is that the door pin switches are independent of each other. While the door trigger wire senses all the doors.
I don't know where its located or what the wire color is for the door trigger wire on a gen 5. So your on your own for finding it. Its a negative trigger (goes to ground whenever any door is open).
So you can tie the ground wire from the indiglo door sill to the door pin switch (for whichever door your installing the indiglo door sills at) or the door trigger wire. The positive wire (power) for the indiglo door sill gets wired to +12V (constant / always hot).
You can probably get away without using a relay, since indiglos don't draw much current. However you can use one if you want. Use a 12V SPST or SPDT automotive relay.
http://nteinc.com/relay_web/R51.html
87: +12V constant
87a: not used (only exists on SPDT relay)
85: ground (door pin switch or door trigger wire)
86: +12V constant
30: output (power wire from indiglo door sill)