I made some custom toolboxes and had to make some changes to my lights so here is what it looks like now.
http://www.truckmodcentral.com/forums/showthread.php?t=6925
http://www.truckmodcentral.com/forums/showthread.php?t=6925
I would still tap the trailer harness. The lights don't pull much power at all from that circuit so its never an issue.
The only thing that I'd do different is buy better light sets than the ones I bought. They were cheap ones I got on ebay from China and one strand has stopped working so I need to fix it.
One bit of advice I have for you, even though the light strands have peel and stick tape on them already, put 3M double sided tape on them to hold them up. The tape that comes on the lights just isn't good enough to make them stick properly.
