what is this part called.....drill a hole to run to the fusebox?

 

I usually use the big rubber grommet for the vehicle harness. No drilling needed, just find it, poke through it with a straightened wire coat hanger, tape your wire to it and pull it through.

 

Exactly what I did

 

I'm trying to do this with my radar detector. I got the "add a circuit" part like in the picture, but when stripping the power cord for my detector, there is a red wire and a white wire. Which one do I connect to the extra fuse spot that goes in the fuse box?

 

90% of companies use the red wire or red striped wire as the hot or +12V lead. That red or red striped wire would be the one that goes to the add-a-circuit wire, the black or black/white wire would just go to a good ground.

To be safe look at the cigarette lighterplug you cut off the radar detector...there will be the same color wires in the plug as the ones you just described. See what color wire is soldered to the tab that the fuse makes contact with...that will be the hot or +12v wire

 

I am assuming you cut off a cig lighter plug. Use a continuity checker to check which wire is connected to the end contact at the point.

 

For the radar detector, pop the hood and put each wire on the battery terminals. If it doesn't work one way, switch the wires. Bingo.

 

If it still dosn't work, that means you had it wrong the first time and you fried the radar detector and need to go buy a new one.
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Verify which wire was connected to the center post on the plug it will need to connect to +.

 

yes make sure you check first. there is no guarantee there is an protection for reverse polarity.