Incidentally the post with the parts I used is in "Trailer towing impressions... " and its Post #11.

You end up putting a connection on each taillight and having to run a wire and fuse back to the battery. The two modules you can put under the panel near the A/C for the battery or you can mount them outside the vehicle, they are supposed to be weathertight and they look like they should be. I had to punch a hole through the rubber plug behind the left taillight to run the wires through and ended up gooping the plug with silicone so water wouldn't get back into the taillight area. Not sure if that was necessary.

If you decide against the isolation unit, it all depends on the trailer. If the power is low enough you'd be okay I guess, but I have no idea what the breakpoint is. Also unfortunately it might not JUST be a warning in your message center... one guy reported that without the isolation when his trailer was hooked on neither of his turn signals would work at all. If he disconnected the trailer the turn signals would work again if I remember right which would imply it was a load issue, but its odd that it would disable the turn signal like that.